diff --git a/dom/animation/Animation.cpp b/dom/animation/Animation.cpp
index dc33c67493..17b01807a5 100644
--- a/dom/animation/Animation.cpp
+++ b/dom/animation/Animation.cpp
@@ -1383,6 +1383,30 @@ Animation::GetRenderedDocument() const
   return mEffect->AsKeyframeEffect()->GetRenderedDocument();
 }
 
+class AsyncFinishNotification : public MicroTaskRunnable
+{
+public:
+  explicit AsyncFinishNotification(Animation* aAnimation)
+  : MicroTaskRunnable()
+  , mAnimation(aAnimation)
+  {}
+
+  virtual void Run(AutoSlowOperation& aAso) override
+  {
+    mAnimation->DoFinishNotificationImmediately(this);
+    mAnimation = nullptr;
+  }
+
+  virtual bool Suppressed() override
+  {
+    nsIGlobalObject* global = mAnimation->GetOwnerGlobal();
+    return global && global->IsInSyncOperation();
+  }
+
+private:
+  RefPtr<Animation> mAnimation;
+};
+
 void
 Animation::DoFinishNotification(SyncNotifyFlag aSyncNotifyFlag)
 {
@@ -1390,9 +1414,8 @@ Animation::DoFinishNotification(SyncNotifyFlag aSyncNotifyFlag)
 
   if (aSyncNotifyFlag == SyncNotifyFlag::Sync) {
     DoFinishNotificationImmediately();
-  } else if (!mFinishNotificationTask.IsPending()) {
-    RefPtr<nsRunnableMethod<Animation>> runnable =
-      NewRunnableMethod(this, &Animation::DoFinishNotificationImmediately);
+  } else if (!mFinishNotificationTask) {
+    RefPtr<MicroTaskRunnable> runnable = new AsyncFinishNotification(this);
     context->DispatchToMicroTask(do_AddRef(runnable));
     mFinishNotificationTask = runnable.forget();
   }
@@ -1415,9 +1438,13 @@ Animation::MaybeResolveFinishedPromise()
 }
 
 void
-Animation::DoFinishNotificationImmediately()
+Animation::DoFinishNotificationImmediately(MicroTaskRunnable* aAsync)
 {
-  mFinishNotificationTask.Revoke();
+  if (aAsync && aAsync != mFinishNotificationTask) {
+    return;
+  }
+
+  mFinishNotificationTask = nullptr;
 
   if (PlayState() != AnimationPlayState::Finished) {
     return;
diff --git a/dom/animation/Animation.h b/dom/animation/Animation.h
index a63579c704..d640dd6ff4 100644
--- a/dom/animation/Animation.h
+++ b/dom/animation/Animation.h
@@ -9,6 +9,7 @@
 #include "nsWrapperCache.h"
 #include "nsCycleCollectionParticipant.h"
 #include "mozilla/Attributes.h"
+#include "mozilla/CycleCollectedJSContext.h"
 #include "mozilla/DOMEventTargetHelper.h"
 #include "mozilla/EffectCompositor.h" // For EffectCompositor::CascadeLevel
 #include "mozilla/LinkedList.h"
@@ -42,6 +43,7 @@ class AnimValuesStyleRule;
 
 namespace dom {
 
+class AsyncFinishNotification;
 class CSSAnimation;
 class CSSTransition;
 
@@ -378,7 +380,8 @@ protected:
   void ResetFinishedPromise();
   void MaybeResolveFinishedPromise();
   void DoFinishNotification(SyncNotifyFlag aSyncNotifyFlag);
-  void DoFinishNotificationImmediately();
+  friend class AsyncFinishNotification;
+  void DoFinishNotificationImmediately(MicroTaskRunnable* aAsync = nullptr);
   void DispatchPlaybackEvent(const nsAString& aName);
 
   /**
@@ -446,7 +449,7 @@ protected:
   // getAnimations() list.
   bool mIsRelevant;
 
-  nsRevocableEventPtr<nsRunnableMethod<Animation>> mFinishNotificationTask;
+  RefPtr<MicroTaskRunnable> mFinishNotificationTask;
   // True if mFinished is resolved or would be resolved if mFinished has
   // yet to be created. This is not set when mFinished is rejected since
   // in that case mFinished is immediately reset to represent a new current
diff --git a/dom/animation/DocumentTimeline.cpp b/dom/animation/DocumentTimeline.cpp
index f6609b5262..2900739f92 100644
--- a/dom/animation/DocumentTimeline.cpp
+++ b/dom/animation/DocumentTimeline.cpp
@@ -155,6 +155,13 @@ DocumentTimeline::WillRefresh(mozilla::TimeStamp aTime)
   bool needsTicks = false;
   nsTArray<Animation*> animationsToRemove(mAnimations.Count());
 
+  // https://drafts.csswg.org/web-animations-1/#update-animations-and-send-events,
+  // step2.
+  // Note that this should be done before nsAutoAnimationMutationBatch.  If
+  // PerformMicroTaskCheckpoint was called before nsAutoAnimationMutationBatch
+  // is destroyed, some mutation records might not be delivered in this
+  // checkpoint.
+  nsAutoMicroTask mt;
   nsAutoAnimationMutationBatch mb(mDocument);
 
   for (Animation* animation = mAnimationOrder.getFirst(); animation;
diff --git a/dom/base/CustomElementRegistry.cpp b/dom/base/CustomElementRegistry.cpp
index 431d78856d..11ba39a67e 100644
--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -5,7 +5,6 @@
 
 #include "mozilla/dom/CustomElementRegistry.h"
 
-#include "mozilla/CycleCollectedJSContext.h"
 #include "mozilla/dom/CustomElementRegistryBinding.h"
 #include "mozilla/dom/HTMLElementBinding.h"
 #include "mozilla/dom/WebComponentsBinding.h"
@@ -1153,7 +1152,7 @@ CustomElementReactionsStack::Enqueue(Element* aElement,
 
   CycleCollectedJSContext* context = CycleCollectedJSContext::Get();
   RefPtr<BackupQueueMicroTask> bqmt = new BackupQueueMicroTask(this);
-  context->DispatchMicroTaskRunnable(bqmt.forget());
+  context->DispatchToMicroTask(bqmt.forget());
 }
 
 void
diff --git a/dom/base/CustomElementRegistry.h b/dom/base/CustomElementRegistry.h
index afbdc2812f..06e5947dbc 100644
--- a/dom/base/CustomElementRegistry.h
+++ b/dom/base/CustomElementRegistry.h
@@ -9,6 +9,7 @@
 #include "js/GCHashTable.h"
 #include "js/TypeDecls.h"
 #include "mozilla/Attributes.h"
+#include "mozilla/CycleCollectedJSContext.h"
 #include "mozilla/ErrorResult.h"
 #include "mozilla/dom/BindingDeclarations.h"
 #include "mozilla/dom/FunctionBinding.h"
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 29cc1bce47..b246132de2 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -5418,10 +5418,10 @@ nsContentUtils::RunInStableState(already_AddRefed<nsIRunnable> aRunnable)
 
 /* static */
 void
-nsContentUtils::RunInMetastableState(already_AddRefed<nsIRunnable> aRunnable)
+nsContentUtils::AddPendingIDBTransaction(already_AddRefed<nsIRunnable> aTransaction)
 {
   MOZ_ASSERT(CycleCollectedJSContext::Get(), "Must be on a script thread!");
-  CycleCollectedJSContext::Get()->RunInMetastableState(Move(aRunnable));
+  CycleCollectedJSContext::Get()->AddPendingIDBTransaction(Move(aTransaction));
 }
 
 /*
@@ -6472,9 +6472,16 @@ nsContentUtils::IsSubDocumentTabbable(nsIContent* aContent)
   contentViewer->GetPreviousViewer(getter_AddRefs(zombieViewer));
 
   // If there are 2 viewers for the current docshell, that
-  // means the current document is a zombie document.
-  // Only navigate into the subdocument if it's not a zombie.
-  return !zombieViewer;
+  // means the current document may be a zombie document.
+  // While load and pageshow events are dispatched, zombie viewer is the old,
+  // to be hidden document.
+  if (zombieViewer) {
+    bool inOnLoad = false;
+    docShell->GetIsExecutingOnLoadHandler(&inOnLoad);
+    return inOnLoad;
+  }
+
+  return true;
 }
 
 bool
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index 6e9f23054a..74239c8036 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -1766,17 +1766,12 @@ public:
    */
   static void RunInStableState(already_AddRefed<nsIRunnable> aRunnable);
 
-  /* Add a "synchronous section", in the form of an nsIRunnable run once the
-   * event loop has reached a "metastable state". |aRunnable| must not cause any
-   * queued events to be processed (i.e. must not spin the event loop).
-   * We've reached a metastable state when the currently executing task or
-   * microtask has finished.  This is not specced at this time.
-   * In practice this runs aRunnable once the currently executing task or
-   * microtask finishes.  If called multiple times per microtask, all the
-   * runnables will be executed, in the order in which RunInMetastableState()
-   * was called
+  /* Add a pending IDBTransaction to be cleaned up at the end of performing a
+   * microtask checkpoint.
+   * See the step of "Cleanup Indexed Database Transactions" in
+   * https://html.spec.whatwg.org/multipage/webappapis.html#perform-a-microtask-checkpoint
    */
-  static void RunInMetastableState(already_AddRefed<nsIRunnable> aRunnable);
+  static void AddPendingIDBTransaction(already_AddRefed<nsIRunnable> aTransaction);
 
   /* Process viewport META data. This gives us information for the scale
    * and zoom of a page on mobile devices. We stick the information in
diff --git a/dom/base/nsDOMMutationObserver.cpp b/dom/base/nsDOMMutationObserver.cpp
index fdad6ee907..732d57c905 100644
--- a/dom/base/nsDOMMutationObserver.cpp
+++ b/dom/base/nsDOMMutationObserver.cpp
@@ -623,7 +623,7 @@ nsDOMMutationObserver::QueueMutationObserverMicroTask()
 
   RefPtr<MutationObserverMicroTask> momt =
     new MutationObserverMicroTask();
-  ccjs->DispatchMicroTaskRunnable(momt.forget());
+  ccjs->DispatchToMicroTask(momt.forget());
 }
 
 void
@@ -644,9 +644,8 @@ nsDOMMutationObserver::RescheduleForRun()
       return;
     }
 
-    RefPtr<MutationObserverMicroTask> momt =
-      new MutationObserverMicroTask();
-    ccjs->DispatchMicroTaskRunnable(momt.forget());
+    RefPtr<MutationObserverMicroTask> momt = new MutationObserverMicroTask();
+    ccjs->DispatchToMicroTask(momt.forget());
     sScheduledMutationObservers = new AutoTArray<RefPtr<nsDOMMutationObserver>, 4>;
   }
 
diff --git a/dom/base/nsGkAtomList.h b/dom/base/nsGkAtomList.h
index af25f97e1c..9622b21d3e 100644
--- a/dom/base/nsGkAtomList.h
+++ b/dom/base/nsGkAtomList.h
@@ -908,6 +908,7 @@ GK_ATOM(onreadsuccess, "onreadsuccess")
 GK_ATOM(onready, "onready")
 GK_ATOM(onreadystatechange, "onreadystatechange")
 GK_ATOM(onreceived, "onreceived")
+GK_ATOM(onrejectionhandled, "onrejectionhandled")
 GK_ATOM(onremoteheld, "onremoteheld")
 GK_ATOM(onremoteresumed, "onremoteresumed")
 GK_ATOM(onrequestprogress, "onrequestprogress")
@@ -951,6 +952,7 @@ GK_ATOM(ontransitionend, "ontransitionend")
 GK_ATOM(ontransitionrun, "ontransitionrun")
 GK_ATOM(ontransitionstart, "ontransitionstart")
 GK_ATOM(onunderflow, "onunderflow")
+GK_ATOM(onunhandledrejection, "onunhandledrejection")
 GK_ATOM(onunload, "onunload")
 GK_ATOM(onupdatefound, "onupdatefound")
 GK_ATOM(onupdateready, "onupdateready")
diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index 2a29ffd09b..da2e5267f0 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -13011,12 +13011,6 @@ nsGlobalWindow::RunTimeoutHandler(Timeout* aTimeout,
   // point anyway, and the script context should have already reported
   // the script error in the usual way - so we just drop it.
 
-  // Since we might be processing more timeouts, go ahead and flush the promise
-  // queue now before we do that.  We need to do that while we're still in our
-  // "running JS is safe" state (e.g. mRunningTimeout is set, timeout->mRunning
-  // is false).
-  Promise::PerformMicroTaskCheckpoint();
-
   if (trackNestingLevel) {
     sNestingLevel = nestingLevel;
   }
diff --git a/dom/base/nsIGlobalObject.cpp b/dom/base/nsIGlobalObject.cpp
index 7a6bab8b4b..a3d22aafa4 100644
--- a/dom/base/nsIGlobalObject.cpp
+++ b/dom/base/nsIGlobalObject.cpp
@@ -145,6 +145,6 @@ void nsIGlobalObject::QueueMicrotask(VoidFunction& aCallback) {
   CycleCollectedJSContext* context = CycleCollectedJSContext::Get();
   if (context) {
     RefPtr<MicroTaskRunnable> mt = new QueuedMicrotask(this, aCallback);
-    context->DispatchMicroTaskRunnable(mt.forget());
+    context->DispatchToMicroTask(mt.forget());
   }
 }
\ No newline at end of file
diff --git a/dom/bindings/BindingUtils.h b/dom/bindings/BindingUtils.h
index 467b134864..4011e8c338 100644
--- a/dom/bindings/BindingUtils.h
+++ b/dom/bindings/BindingUtils.h
@@ -13,7 +13,6 @@
 #include "mozilla/Alignment.h"
 #include "mozilla/Array.h"
 #include "mozilla/Assertions.h"
-#include "mozilla/CycleCollectedJSContext.h"
 #include "mozilla/DeferredFinalize.h"
 #include "mozilla/dom/BindingDeclarations.h"
 #include "mozilla/dom/CallbackObject.h"
diff --git a/dom/bindings/CallbackObject.cpp b/dom/bindings/CallbackObject.cpp
index eb4b828ffe..3034f2b019 100644
--- a/dom/bindings/CallbackObject.cpp
+++ b/dom/bindings/CallbackObject.cpp
@@ -78,11 +78,9 @@ CallbackObject::CallSetup::CallSetup(CallbackObject* aCallback,
   , mExceptionHandling(aExceptionHandling)
   , mIsMainThread(NS_IsMainThread())
 {
-  if (mIsMainThread) {
-    CycleCollectedJSContext* ccjs = CycleCollectedJSContext::Get();
-    if (ccjs) {
-      ccjs->EnterMicroTask();
-    }
+  CycleCollectedJSContext* ccjs = CycleCollectedJSContext::Get();
+  if (ccjs) {
+    ccjs->EnterMicroTask();
   }
 
   // Compute the caller's subject principal (if necessary) early, before we
@@ -290,11 +288,9 @@ CallbackObject::CallSetup::~CallSetup()
 
   // It is important that this is the last thing we do, after leaving the
   // compartment and undoing all our entry/incumbent script changes
-  if (mIsMainThread) {
-    CycleCollectedJSContext* ccjs = CycleCollectedJSContext::Get();
-    if (ccjs) {
-      ccjs->LeaveMicroTask();
-    }
+  CycleCollectedJSContext* ccjs = CycleCollectedJSContext::Get();
+  if (ccjs) {
+    ccjs->LeaveMicroTask();
   }
 }
 
diff --git a/dom/bindings/Codegen.py b/dom/bindings/Codegen.py
index b7caaad7bb..3094a98c41 100644
--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -80,7 +80,7 @@ def idlTypeNeedsCycleCollection(type):
         type.isObject() or
         type.isSpiderMonkeyInterface()):
         return False
-    elif type.isCallback() or type.isGeckoInterface():
+    elif type.isCallback() or type.isPromise() or type.isGeckoInterface():
         return True
     elif type.isUnion():
         return any(idlTypeNeedsCycleCollection(t) for t in type.flatMemberTypes)
diff --git a/dom/events/EventListenerManager.cpp b/dom/events/EventListenerManager.cpp
index b6fbf6f6ad..02558638d1 100644
--- a/dom/events/EventListenerManager.cpp
+++ b/dom/events/EventListenerManager.cpp
@@ -1065,12 +1065,8 @@ EventListenerManager::HandleEventSubType(Listener* aListener,
   }
 
   if (NS_SUCCEEDED(result)) {
-    if (mIsMainThreadELM) {
-      CycleCollectedJSContext* ccjs = CycleCollectedJSContext::Get();
-      if (ccjs) {
-        ccjs->EnterMicroTask();
-      }
-    }
+    nsAutoMicroTask mt;
+
     // nsIDOMEvent::currentTarget is set in EventDispatcher.
     if (listenerHolder.HasWebIDLCallback()) {
       ErrorResult rv;
@@ -1080,12 +1076,6 @@ EventListenerManager::HandleEventSubType(Listener* aListener,
     } else {
       result = listenerHolder.GetXPCOMCallback()->HandleEvent(aDOMEvent);
     }
-    if (mIsMainThreadELM) {
-      CycleCollectedJSContext* ccjs = CycleCollectedJSContext::Get();
-      if (ccjs) {
-        ccjs->LeaveMicroTask();
-      }
-    }
   }
 
   return result;
diff --git a/dom/events/EventNameList.h b/dom/events/EventNameList.h
index c4edc1fe2a..0a17b5f147 100644
--- a/dom/events/EventNameList.h
+++ b/dom/events/EventNameList.h
@@ -578,14 +578,14 @@ WINDOW_EVENT(popstate,
              ePopState,
              EventNameType_XUL | EventNameType_HTMLBodyOrFramesetOnly,
              eBasicEventClass)
-// Not supported yet
-// WINDOW_EVENT(redo)
+WINDOW_EVENT(rejectionhandled, eRejectionHandled,
+             EventNameType_HTMLBodyOrFramesetOnly, eBasicEventClass)
 WINDOW_EVENT(storage,
              eStorage,
              EventNameType_HTMLBodyOrFramesetOnly,
              eBasicEventClass)
-// Not supported yet
-// WINDOW_EVENT(undo)
+WINDOW_EVENT(unhandledrejection, eUnhandledRejection,
+             EventNameType_HTMLBodyOrFramesetOnly, eBasicEventClass)
 WINDOW_EVENT(unload,
              eUnload,
              (EventNameType_XUL | EventNameType_SVGSVG |
diff --git a/dom/geolocation/nsGeolocation.cpp b/dom/geolocation/nsGeolocation.cpp
index 0b447dfa7a..826af388ba 100644
--- a/dom/geolocation/nsGeolocation.cpp
+++ b/dom/geolocation/nsGeolocation.cpp
@@ -18,10 +18,11 @@
 #include "nsIObserverService.h"
 #include "nsPIDOMWindow.h"
 #include "nsThreadUtils.h"
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/CycleCollectedJSContext.h"
+#include "mozilla/Preferences.h"
 #include "mozilla/Services.h"
 #include "mozilla/Unused.h"
-#include "mozilla/Preferences.h"
-#include "mozilla/ClearOnShutdown.h"
 #include "mozilla/WeakPtr.h"
 #include "mozilla/dom/PermissionMessageUtils.h"
 
diff --git a/dom/indexedDB/ActorsChild.cpp b/dom/indexedDB/ActorsChild.cpp
index 31437ea059..6ec99e58f6 100644
--- a/dom/indexedDB/ActorsChild.cpp
+++ b/dom/indexedDB/ActorsChild.cpp
@@ -18,6 +18,7 @@
 #include "IndexedDatabase.h"
 #include "IndexedDatabaseInlines.h"
 #include "mozilla/BasicEvents.h"
+#include "mozilla/CycleCollectedJSContext.h"
 #include "mozilla/Maybe.h"
 #include "mozilla/TypeTraits.h"
 #include "mozilla/dom/Element.h"
@@ -870,22 +871,26 @@ DispatchSuccessEvent(ResultHelper* aResultHelper,
                  IDB_LOG_STRINGIFY(aEvent, kSuccessEventType));
   }
 
+  MOZ_ASSERT_IF(transaction,
+                transaction->IsOpen() && !transaction->IsAborted());
+
   bool dummy;
   nsresult rv = request->DispatchEvent(aEvent, &dummy);
   if (NS_WARN_IF(NS_FAILED(rv))) {
     return;
   }
 
-  MOZ_ASSERT_IF(transaction,
-                transaction->IsOpen() || transaction->IsAborted());
-
   WidgetEvent* internalEvent = aEvent->WidgetEventPtr();
   MOZ_ASSERT(internalEvent);
 
   if (transaction &&
-      transaction->IsOpen() &&
-      internalEvent->mFlags.mExceptionWasRaised) {
-    transaction->Abort(NS_ERROR_DOM_INDEXEDDB_ABORT_ERR);
+      transaction->IsOpen()) {
+    if (internalEvent->mFlags.mExceptionWasRaised) {
+      transaction->Abort(NS_ERROR_DOM_INDEXEDDB_ABORT_ERR);
+    } else {
+      // To handle upgrade transaction.
+      transaction->Run();
+    }
   }
 }
 
diff --git a/dom/indexedDB/ActorsParent.cpp b/dom/indexedDB/ActorsParent.cpp
index 0c6bd1ee1d..d80b900e76 100644
--- a/dom/indexedDB/ActorsParent.cpp
+++ b/dom/indexedDB/ActorsParent.cpp
@@ -23,6 +23,7 @@
 #include "mozilla/AutoRestore.h"
 #include "mozilla/Casting.h"
 #include "mozilla/CheckedInt.h"
+#include "mozilla/CycleCollectedJSContext.h"
 #include "mozilla/EndianUtils.h"
 #include "mozilla/ErrorNames.h"
 #include "mozilla/LazyIdleThread.h"
diff --git a/dom/indexedDB/IDBFileHandle.cpp b/dom/indexedDB/IDBFileHandle.cpp
index 823e643998..3ee284386c 100644
--- a/dom/indexedDB/IDBFileHandle.cpp
+++ b/dom/indexedDB/IDBFileHandle.cpp
@@ -55,7 +55,7 @@ IDBFileHandle::Create(IDBMutableFile* aMutableFile,
   MOZ_ASSERT(NS_IsMainThread(), "This won't work on non-main threads!");
 
   nsCOMPtr<nsIRunnable> runnable = do_QueryObject(fileHandle);
-  nsContentUtils::RunInMetastableState(runnable.forget());
+  nsContentUtils::AddPendingIDBTransaction(runnable.forget());
 
   fileHandle->SetCreating();
 
diff --git a/dom/indexedDB/IDBTransaction.cpp b/dom/indexedDB/IDBTransaction.cpp
index 4ee6239a25..8034588d31 100644
--- a/dom/indexedDB/IDBTransaction.cpp
+++ b/dom/indexedDB/IDBTransaction.cpp
@@ -190,13 +190,9 @@ IDBTransaction::CreateVersionChange(
 
   transaction->SetScriptOwner(aDatabase->GetScriptOwner());
 
-  nsCOMPtr<nsIRunnable> runnable = do_QueryObject(transaction);
-  nsContentUtils::RunInMetastableState(runnable.forget());
-
   transaction->mBackgroundActor.mVersionChangeBackgroundActor = aActor;
   transaction->mNextObjectStoreId = aNextObjectStoreId;
   transaction->mNextIndexId = aNextIndexId;
-  transaction->mCreating = true;
 
   aDatabase->RegisterTransaction(transaction);
   transaction->mRegistered = true;
@@ -226,7 +222,7 @@ IDBTransaction::Create(JSContext* aCx, IDBDatabase* aDatabase,
   transaction->SetScriptOwner(aDatabase->GetScriptOwner());
 
   nsCOMPtr<nsIRunnable> runnable = do_QueryObject(transaction);
-  nsContentUtils::RunInMetastableState(runnable.forget());
+  nsContentUtils::AddPendingIDBTransaction(runnable.forget());
 
   transaction->mCreating = true;
 
diff --git a/dom/plugins/base/nsNPAPIPlugin.cpp b/dom/plugins/base/nsNPAPIPlugin.cpp
index 4b02440e20..5f138a6c71 100644
--- a/dom/plugins/base/nsNPAPIPlugin.cpp
+++ b/dom/plugins/base/nsNPAPIPlugin.cpp
@@ -22,6 +22,7 @@
 #include "nsPluginStreamListenerPeer.h"
 #include "nsIServiceManager.h"
 #include "nsThreadUtils.h"
+#include "mozilla/CycleCollectedJSContext.h"
 #include "mozilla/Preferences.h"
 #include "nsPluginInstanceOwner.h"
 
diff --git a/dom/promise/Promise.cpp b/dom/promise/Promise.cpp
index 0e1349d087..fbaa8e1eaa 100644
--- a/dom/promise/Promise.cpp
+++ b/dom/promise/Promise.cpp
@@ -508,110 +508,6 @@ Promise::ReportRejectedPromise(JSContext* aCx, JS::HandleObject aPromise)
   NS_DispatchToMainThread(new AsyncErrorReporter(xpcReport));
 }
 
-bool
-Promise::PerformMicroTaskCheckpoint()
-{
-  MOZ_ASSERT(NS_IsMainThread(), "Wrong thread!");
-
-  CycleCollectedJSContext* context = CycleCollectedJSContext::Get();
-
-  // On the main thread, we always use the main promise micro task queue.
-  std::queue<nsCOMPtr<nsIRunnable>>& microtaskQueue =
-    context->GetPromiseMicroTaskQueue();
-
-  if (microtaskQueue.empty()) {
-    return false;
-  }
-
-  AutoSlowOperation aso;
-
-  do {
-    nsCOMPtr<nsIRunnable> runnable = microtaskQueue.front().forget();
-    MOZ_ASSERT(runnable);
-
-    // This function can re-enter, so we remove the element before calling.
-    microtaskQueue.pop();
-    nsresult rv = runnable->Run();
-    if (NS_WARN_IF(NS_FAILED(rv))) {
-      return false;
-    }
-    aso.CheckForInterrupt();
-    context->AfterProcessMicrotask();
-  } while (!microtaskQueue.empty());
-
-  return true;
-}
-
-void
-Promise::PerformWorkerMicroTaskCheckpoint()
-{
-  MOZ_ASSERT(!NS_IsMainThread(), "Wrong thread!");
-
-  CycleCollectedJSContext* context = CycleCollectedJSContext::Get();
-  if (!context) {
-    return;
-  }
-
-  for (;;) {
-    // For a normal microtask checkpoint, we try to use the debugger microtask
-    // queue first. If the debugger queue is empty, we use the normal microtask
-    // queue instead.
-    std::queue<nsCOMPtr<nsIRunnable>>* microtaskQueue =
-      &context->GetDebuggerPromiseMicroTaskQueue();
-
-    if (microtaskQueue->empty()) {
-      microtaskQueue = &context->GetPromiseMicroTaskQueue();
-      if (microtaskQueue->empty()) {
-        break;
-      }
-    }
-
-    nsCOMPtr<nsIRunnable> runnable = microtaskQueue->front().forget();
-    MOZ_ASSERT(runnable);
-
-    // This function can re-enter, so we remove the element before calling.
-    microtaskQueue->pop();
-    nsresult rv = runnable->Run();
-    if (NS_WARN_IF(NS_FAILED(rv))) {
-      return;
-    }
-    context->AfterProcessMicrotask();
-  }
-}
-
-void
-Promise::PerformWorkerDebuggerMicroTaskCheckpoint()
-{
-  MOZ_ASSERT(!NS_IsMainThread(), "Wrong thread!");
-
-  CycleCollectedJSContext* context = CycleCollectedJSContext::Get();
-  if (!context) {
-    return;
-  }
-
-  for (;;) {
-    // For a debugger microtask checkpoint, we always use the debugger microtask
-    // queue.
-    std::queue<nsCOMPtr<nsIRunnable>>* microtaskQueue =
-      &context->GetDebuggerPromiseMicroTaskQueue();
-
-    if (microtaskQueue->empty()) {
-      break;
-    }
-
-    nsCOMPtr<nsIRunnable> runnable = microtaskQueue->front().forget();
-    MOZ_ASSERT(runnable);
-
-    // This function can re-enter, so we remove the element before calling.
-    microtaskQueue->pop();
-    nsresult rv = runnable->Run();
-    if (NS_WARN_IF(NS_FAILED(rv))) {
-      return;
-    }
-    context->AfterProcessMicrotask();
-  }
-}
-
 JSObject*
 Promise::GlobalJSObject() const
 {
diff --git a/dom/promise/Promise.h b/dom/promise/Promise.h
index 31f7bcae9e..dfbb82d489 100644
--- a/dom/promise/Promise.h
+++ b/dom/promise/Promise.h
@@ -104,14 +104,6 @@ public:
                                            // specializations in the .cpp for
                                            // the T values we support.
 
-  // Called by DOM to let us execute our callbacks.  May be called recursively.
-  // Returns true if at least one microtask was processed.
-  static bool PerformMicroTaskCheckpoint();
-
-  static void PerformWorkerMicroTaskCheckpoint();
-
-  static void PerformWorkerDebuggerMicroTaskCheckpoint();
-
   // WebIDL
 
   nsIGlobalObject* GetParentObject() const
diff --git a/dom/promise/PromiseDebugging.cpp b/dom/promise/PromiseDebugging.cpp
index 14ad69bdc8..c0f3f534dc 100644
--- a/dom/promise/PromiseDebugging.cpp
+++ b/dom/promise/PromiseDebugging.cpp
@@ -217,8 +217,16 @@ PromiseDebugging::RemoveUncaughtRejectionObserver(GlobalObject&,
 /* static */ void
 PromiseDebugging::AddUncaughtRejection(JS::HandleObject aPromise)
 {
+  CycleCollectedJSContext* storage = CycleCollectedJSContext::Get();
+  auto& uncaught = storage->mUncaughtRejections;
+  auto& outstanding = storage->mOutstandingRejections;
+
+  // See 8.1.4.7 Unhandled promise rejections, Step 5.1.4.
+  RefPtr<Promise> promise = Promise::CreateFromExisting(xpc::NativeGlobal(aPromise), aPromise);
+  uint64_t promiseID = JS::GetPromiseID(aPromise);
+  outstanding.Put(promiseID, promise);
   // This might OOM, but won't set a pending exception, so we'll just ignore it.
-  if (CycleCollectedJSContext::Get()->mUncaughtRejections.append(aPromise)) {
+  if (uncaught.append(aPromise)) {
     FlushRejections::DispatchNeeded();
   }
 }
@@ -226,15 +234,20 @@ PromiseDebugging::AddUncaughtRejection(JS::HandleObject aPromise)
 /* void */ void
 PromiseDebugging::AddConsumedRejection(JS::HandleObject aPromise)
 {
+  CycleCollectedJSContext* storage = CycleCollectedJSContext::Get();
+  auto& uncaught = storage->mUncaughtRejections;
+  auto& outstanding = storage->mOutstandingRejections;
+
   // If the promise is in our list of uncaught rejections, we haven't yet
   // reported it as unhandled. In that case, just remove it from the list
   // and don't add it to the list of consumed rejections.
-  auto& uncaughtRejections = CycleCollectedJSContext::Get()->mUncaughtRejections;
-  for (size_t i = 0; i < uncaughtRejections.length(); i++) {
-    if (uncaughtRejections[i] == aPromise) {
+  uint64_t promiseID = JS::GetPromiseID(aPromise);
+  outstanding.Remove(promiseID);
+  for (size_t i = 0; i < uncaught.length(); i++) {
+    if (uncaught[i] == aPromise) {
       // To avoid large amounts of memmoves, we don't shrink the vector here.
       // Instead, we filter out nullptrs when iterating over the vector later.
-      uncaughtRejections[i].set(nullptr);
+      uncaught[i].set(nullptr);
       return;
     }
   }
@@ -251,6 +264,7 @@ PromiseDebugging::FlushUncaughtRejectionsInternal()
 
   auto& uncaught = storage->mUncaughtRejections;
   auto& consumed = storage->mConsumedRejections;
+  auto& outstanding = storage->mOutstandingRejections;
 
   AutoJSAPI jsapi;
   jsapi.Init();
@@ -267,6 +281,10 @@ PromiseDebugging::FlushUncaughtRejectionsInternal()
       continue;
     }
 
+    // Clean up outstanding rejected promises weak set
+    uint64_t promiseID = JS::GetPromiseID(promise);
+    outstanding.Remove(promiseID);
+
     for (size_t j = 0; j < observers.Length(); ++j) {
       RefPtr<UncaughtRejectionObserver> obs =
         static_cast<UncaughtRejectionObserver*>(observers[j].get());
@@ -274,8 +292,12 @@ PromiseDebugging::FlushUncaughtRejectionsInternal()
       IgnoredErrorResult err;
       obs->OnLeftUncaught(promise, err);
     }
-    JSAutoCompartment ac(cx, promise);
-    Promise::ReportRejectedPromise(cx, promise);
+    // report error to console, unless marked by unhandledrejection event
+    bool reportError = !JS::GetPromiseIsReported(promise);
+    if (reportError) {
+      JSAutoCompartment ac(cx, promise);
+      Promise::ReportRejectedPromise(cx, promise);
+    }
   }
   storage->mUncaughtRejections.clear();
 
diff --git a/dom/script/ScriptLoader.cpp b/dom/script/ScriptLoader.cpp
index 0750e03db7..7264390939 100644
--- a/dom/script/ScriptLoader.cpp
+++ b/dom/script/ScriptLoader.cpp
@@ -52,6 +52,7 @@
 #include "ImportManager.h"
 #include "mozilla/dom/EncodingUtils.h"
 #include "mozilla/ConsoleReportCollector.h"
+#include "mozilla/CycleCollectedJSContext.h"
 
 #include "mozilla/Attributes.h"
 #include "mozilla/Unused.h"
diff --git a/dom/script/ScriptSettings.cpp b/dom/script/ScriptSettings.cpp
index 790394de65..d315243531 100644
--- a/dom/script/ScriptSettings.cpp
+++ b/dom/script/ScriptSettings.cpp
@@ -828,8 +828,6 @@ AutoSafeJSContext::AutoSafeJSContext(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_IN_IMP
 AutoSlowOperation::AutoSlowOperation(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_IN_IMPL)
   : AutoJSAPI()
 {
-  MOZ_ASSERT(NS_IsMainThread());
-
   MOZ_GUARD_OBJECT_NOTIFIER_INIT;
 
   Init();
@@ -838,9 +836,12 @@ AutoSlowOperation::AutoSlowOperation(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_IN_IMP
 void
 AutoSlowOperation::CheckForInterrupt()
 {
-  // JS_CheckForInterrupt expects us to be in a compartment.
-  JSAutoCompartment ac(cx(), xpc::UnprivilegedJunkScope());
-  JS_CheckForInterrupt(cx());
+  // For now we support only main thread!
+  if (mIsMainThread) {
+    // JS_CheckForInterrupt expects us to be in a compartment.
+    JSAutoCompartment ac(cx(), xpc::UnprivilegedJunkScope());
+    JS_CheckForInterrupt(cx());
+  }
 }
 
 } // namespace mozilla
diff --git a/dom/script/ScriptSettings.h b/dom/script/ScriptSettings.h
index f2e12f0be9..11181dda36 100644
--- a/dom/script/ScriptSettings.h
+++ b/dom/script/ScriptSettings.h
@@ -298,7 +298,6 @@ protected:
   // AutoJSAPI, so Init must NOT be called on subclasses that use this.
   AutoJSAPI(nsIGlobalObject* aGlobalObject, bool aIsMainThread, Type aType);
 
-private:
   mozilla::Maybe<JSAutoRequest> mAutoRequest;
   mozilla::Maybe<JSAutoNullableCompartment> mAutoNullableCompartment;
   JSContext *mCx;
@@ -307,6 +306,7 @@ private:
   bool mIsMainThread;
   Maybe<JS::WarningReporter> mOldWarningReporter;
 
+private:
   void InitInternal(nsIGlobalObject* aGlobalObject, JSObject* aGlobal,
                     JSContext* aCx, bool aIsMainThread);
 
diff --git a/dom/webidl/EventHandler.webidl b/dom/webidl/EventHandler.webidl
index f7acb66ef4..853067bd6f 100644
--- a/dom/webidl/EventHandler.webidl
+++ b/dom/webidl/EventHandler.webidl
@@ -160,7 +160,9 @@ interface WindowEventHandlers {
            attribute EventHandler onpagehide;
            attribute EventHandler onpageshow;
            attribute EventHandler onpopstate;
+           attribute EventHandler onrejectionhandled;
            attribute EventHandler onstorage;
+           attribute EventHandler onunhandledrejection;
            attribute EventHandler onunload;
 };
 
diff --git a/dom/webidl/PromiseRejectionEvent.webidl b/dom/webidl/PromiseRejectionEvent.webidl
new file mode 100644
index 0000000000..3c489f3db4
--- /dev/null
+++ b/dom/webidl/PromiseRejectionEvent.webidl
@@ -0,0 +1,22 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * The origin of this IDL file is
+ * https://html.spec.whatwg.org/multipage/webappapis.html#the-promiserejectionevent-interface
+ */
+
+[Constructor(DOMString type, PromiseRejectionEventInit eventInitDict),
+ Exposed=(Window,Worker)]
+interface PromiseRejectionEvent : Event
+{
+  [BinaryName="rejectedPromise"]
+  readonly attribute Promise<any> promise;
+  readonly attribute any reason;
+};
+
+dictionary PromiseRejectionEventInit : EventInit {
+  required Promise<any> promise;
+           any reason;
+};
diff --git a/dom/webidl/WorkerGlobalScope.webidl b/dom/webidl/WorkerGlobalScope.webidl
index 25cfe9e294..7afba6c367 100644
--- a/dom/webidl/WorkerGlobalScope.webidl
+++ b/dom/webidl/WorkerGlobalScope.webidl
@@ -25,6 +25,8 @@ interface WorkerGlobalScope : EventTarget {
 
   attribute EventHandler onoffline;
   attribute EventHandler ononline;
+  attribute EventHandler onrejectionhandled;
+  attribute EventHandler onunhandledrejection;
   // also has additional members in a partial interface
 };
 
diff --git a/dom/webidl/moz.build b/dom/webidl/moz.build
index 6e7871efa0..4fc43b275f 100644
--- a/dom/webidl/moz.build
+++ b/dom/webidl/moz.build
@@ -691,6 +691,7 @@ GENERATED_EVENTS_WEBIDL_FILES = [
     'PopStateEvent.webidl',
     'PopupBlockedEvent.webidl',
     'ProgressEvent.webidl',
+    'PromiseRejectionEvent.webidl',
     'RecordErrorEvent.webidl',
     'ScrollViewChangeEvent.webidl',
     'ServiceWorkerMessageEvent.webidl',
diff --git a/dom/workers/RuntimeService.cpp b/dom/workers/RuntimeService.cpp
index 6138a3c6de..199f6ea56a 100644
--- a/dom/workers/RuntimeService.cpp
+++ b/dom/workers/RuntimeService.cpp
@@ -1007,6 +1007,10 @@ public:
     : mWorkerPrivate(aWorkerPrivate)
   {
     MOZ_ASSERT(aWorkerPrivate);
+    // Magical number 2. Workers have the base recursion depth 1, and normal
+    // runnables run at level 2, and we don't want to process microtasks
+    // at any other level.
+    SetTargetedMicroTaskRecursionDepth(2);
   }
 
   ~WorkerJSContext()
@@ -1092,26 +1096,14 @@ public:
     }
   }
 
-  virtual void AfterProcessTask(uint32_t aRecursionDepth) override
+  virtual void DispatchToMicroTask(already_AddRefed<MicroTaskRunnable> aRunnable) override
   {
-    // Only perform the Promise microtask checkpoint on the outermost event
-    // loop.  Don't run it, for example, during sync XHR or importScripts.
-    if (aRecursionDepth == 2) {
-      CycleCollectedJSContext::AfterProcessTask(aRecursionDepth);
-    } else if (aRecursionDepth > 2) {
-      AutoDisableMicroTaskCheckpoint disableMicroTaskCheckpoint;
-      CycleCollectedJSContext::AfterProcessTask(aRecursionDepth);
-    }
-  }
-
-  virtual void DispatchToMicroTask(already_AddRefed<nsIRunnable> aRunnable) override
-  {
-    RefPtr<nsIRunnable> runnable(aRunnable);
+    RefPtr<MicroTaskRunnable> runnable(aRunnable);
 
     MOZ_ASSERT(!NS_IsMainThread());
     MOZ_ASSERT(runnable);
 
-    std::queue<nsCOMPtr<nsIRunnable>>* microTaskQueue = nullptr;
+    std::queue<RefPtr<MicroTaskRunnable>>* microTaskQueue = nullptr;
 
     JSContext* cx = GetCurrentThreadJSContext();
     NS_ASSERTION(cx, "This should never be null!");
@@ -1120,15 +1112,15 @@ public:
     NS_ASSERTION(global, "This should never be null!");
 
     // On worker threads, if the current global is the worker global, we use the
-    // main promise micro task queue. Otherwise, the current global must be
+    // main micro task queue. Otherwise, the current global must be
     // either the debugger global or a debugger sandbox, and we use the debugger
-    // promise micro task queue instead.
+    // micro task queue instead.
     if (IsWorkerGlobal(global)) {
-      microTaskQueue = &mPromiseMicroTaskQueue;
+      microTaskQueue = &GetMicroTaskQueue();
     } else {
       MOZ_ASSERT(IsDebuggerGlobal(global) || IsDebuggerSandbox(global));
 
-      microTaskQueue = &mDebuggerPromiseMicroTaskQueue;
+      microTaskQueue = &GetDebuggerMicroTaskQueue();
     }
 
     microTaskQueue->push(runnable.forget());
diff --git a/dom/workers/WorkerPrivate.cpp b/dom/workers/WorkerPrivate.cpp
index 7b35695d08..eccdf77fc4 100644
--- a/dom/workers/WorkerPrivate.cpp
+++ b/dom/workers/WorkerPrivate.cpp
@@ -40,6 +40,7 @@
 #include "mozilla/Assertions.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/ContentEvents.h"
+#include "mozilla/CycleCollectedJSContext.h"
 #include "mozilla/EventDispatcher.h"
 #include "mozilla/Likely.h"
 #include "mozilla/LoadContext.h"
@@ -4815,8 +4816,10 @@ WorkerPrivate::DoRunLoop(JSContext* aCx)
       static_cast<nsIRunnable*>(runnable)->Run();
       runnable->Release();
 
-      // Flush the promise queue.
-      Promise::PerformWorkerDebuggerMicroTaskCheckpoint();
+      CycleCollectedJSContext* ccjs = CycleCollectedJSContext::Get();
+      if (ccjs) {
+        ccjs->PerformDebuggerMicroTaskCheckpoint();
+      }
 
       if (debuggerRunnablesPending) {
         WorkerDebuggerGlobalScope* globalScope = DebuggerGlobalScope();
@@ -5802,8 +5805,12 @@ WorkerPrivate::EnterDebuggerEventLoop()
     {
       MutexAutoLock lock(mMutex);
 
+      CycleCollectedJSContext* context = CycleCollectedJSContext::Get();
+      std::queue<RefPtr<MicroTaskRunnable>>& debuggerMtQueue =
+        context->GetDebuggerMicroTaskQueue();
       while (mControlQueue.IsEmpty() &&
-             !(debuggerRunnablesPending = !mDebuggerQueue.IsEmpty())) {
+             !(debuggerRunnablesPending = !mDebuggerQueue.IsEmpty()) && 
+             debuggerMtQueue.empty()) {
         WaitForWorkerEvents();
       }
 
@@ -5812,6 +5819,11 @@ WorkerPrivate::EnterDebuggerEventLoop()
       // XXXkhuey should we abort JS on the stack here if we got Abort above?
     }
 
+    CycleCollectedJSContext* context = CycleCollectedJSContext::Get();
+    if (context) {
+      context->PerformDebuggerMicroTaskCheckpoint();
+    }
+
     if (debuggerRunnablesPending) {
       // Start the periodic GC timer if it is not already running.
       SetGCTimerMode(PeriodicTimer);
@@ -5828,8 +5840,10 @@ WorkerPrivate::EnterDebuggerEventLoop()
       static_cast<nsIRunnable*>(runnable)->Run();
       runnable->Release();
 
-      // Flush the promise queue.
-      Promise::PerformWorkerDebuggerMicroTaskCheckpoint();
+      CycleCollectedJSContext* ccjs = CycleCollectedJSContext::Get();
+      if (ccjs) {
+        ccjs->PerformDebuggerMicroTaskCheckpoint();
+      }
 
       // Now *might* be a good time to GC. Let the JS engine make the decision.
       if (JS::CurrentGlobalOrNull(cx)) {
@@ -6256,8 +6270,8 @@ WorkerPrivate::RunExpiredTimeouts(JSContext* aCx)
 
     RefPtr<Function> callback = info->mHandler->GetCallback();
     if (!callback) {
-      // scope for the AutoEntryScript, so it comes off the stack before we do
-      // Promise::PerformMicroTaskCheckpoint.
+      nsAutoMicroTask mt;
+
       AutoEntryScript aes(global, reason, false);
 
       // Evaluate the timeout expression.
@@ -6292,10 +6306,6 @@ WorkerPrivate::RunExpiredTimeouts(JSContext* aCx)
       rv.SuppressException();
     }
 
-    // Since we might be processing more timeouts, go ahead and flush
-    // the promise queue now before we do that.
-    Promise::PerformWorkerMicroTaskCheckpoint();
-
     NS_ASSERTION(mRunningExpiredTimeouts, "Someone changed this!");
   }
 
diff --git a/dom/workers/WorkerScope.h b/dom/workers/WorkerScope.h
index df58c874f1..7a4c3c8bb2 100644
--- a/dom/workers/WorkerScope.h
+++ b/dom/workers/WorkerScope.h
@@ -151,6 +151,8 @@ public:
 
   IMPL_EVENT_HANDLER(online)
   IMPL_EVENT_HANDLER(offline)
+  IMPL_EVENT_HANDLER(rejectionhandled)
+  IMPL_EVENT_HANDLER(unhandledrejection)
 
   void
   Dump(const Optional<nsAString>& aString) const;
diff --git a/js/src/builtin/Promise.h b/js/src/builtin/Promise.h
index 5e5c850d60..53769ef0bf 100644
--- a/js/src/builtin/Promise.h
+++ b/js/src/builtin/Promise.h
@@ -100,6 +100,10 @@ class PromiseObject : public NativeObject
         int32_t flags = getFixedSlot(PromiseSlot_Flags).toInt32();
         setFixedSlot(PromiseSlot_Flags, Int32Value(flags | PROMISE_FLAG_REPORTED));
     }
+    bool isReported() {
+        MOZ_ASSERT(isUnhandled());
+        return flags() & PROMISE_FLAG_REPORTED;
+    }
 };
 
 /**
diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index 1be4e39113..5aa38917f5 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -4949,12 +4949,31 @@ JS::GetPromiseResult(JS::HandleObject promiseObj)
     return promise->state() == JS::PromiseState::Fulfilled ? promise->value() : promise->reason();
 }
 
+JS_PUBLIC_API(bool)
+JS::GetPromiseIsHandled(JS::HandleObject promise)
+{
+    PromiseObject* promiseObj = &promise->as<PromiseObject>();
+    return !promiseObj->isUnhandled();
+}
+
 JS_PUBLIC_API(JSObject*)
 JS::GetPromiseAllocationSite(JS::HandleObject promise)
 {
     return promise->as<PromiseObject>().allocationSite();
 }
 
+JS_PUBLIC_API(bool)
+JS::GetPromiseIsReported(JS::HandleObject promise)
+{
+    return promise->as<PromiseObject>().isReported();
+}
+
+JS_PUBLIC_API(void)
+JS::MarkPromiseRejectionReported(JS::HandleObject promise)
+{
+    return promise->as<PromiseObject>().markAsReported();
+}
+
 JS_PUBLIC_API(JSObject*)
 JS::GetPromiseResolutionSite(JS::HandleObject promise)
 {
diff --git a/js/src/jsapi.h b/js/src/jsapi.h
index 444b9e2ed2..f1e71f9e7a 100644
--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -4611,6 +4611,27 @@ GetPromiseID(JS::HandleObject promise);
 extern JS_PUBLIC_API(JS::Value)
 GetPromiseResult(JS::HandleObject promise);
 
+/**
+ * Returns whether the given promise's rejection is already handled or not.
+ *
+ * The caller must check the given promise is rejected before checking it's
+ * handled or not.
+ */
+extern JS_PUBLIC_API(bool)
+GetPromiseIsHandled(JS::HandleObject promise);
+
+/**
+ * Returns whether the given promise's rejection is reported or not.
+ */
+extern JS_PUBLIC_API(bool)
+GetPromiseIsReported(JS::HandleObject promise);
+
+/**
+ * Mark the given promise's rejection as already reported.
+ */
+extern JS_PUBLIC_API(void)
+MarkPromiseRejectionReported(JS::HandleObject promise);
+
 /**
  * Returns a js::SavedFrame linked list of the stack that lead to the given
  * Promise's allocation.
diff --git a/js/xpconnect/src/XPCJSContext.cpp b/js/xpconnect/src/XPCJSContext.cpp
index be9a75f799..d45f9f49eb 100644
--- a/js/xpconnect/src/XPCJSContext.cpp
+++ b/js/xpconnect/src/XPCJSContext.cpp
@@ -3485,21 +3485,6 @@ XPCJSContext::BeforeProcessTask(bool aMightBlock)
 {
     MOZ_ASSERT(NS_IsMainThread());
 
-    // If ProcessNextEvent was called during a Promise "then" callback, we
-    // must process any pending microtasks before blocking in the event loop,
-    // otherwise we may deadlock until an event enters the queue later.
-    if (aMightBlock) {
-        if (Promise::PerformMicroTaskCheckpoint()) {
-            // If any microtask was processed, we post a dummy event in order to
-            // force the ProcessNextEvent call not to block.  This is required
-            // to support nested event loops implemented using a pattern like
-            // "while (condition) thread.processNextEvent(true)", in case the
-            // condition is triggered here by a Promise "then" callback.
-
-            NS_DispatchToMainThread(new Runnable());
-        }
-    }
-
     // Start the slow script timer.
     mSlowScriptCheckpoint = mozilla::TimeStamp::NowLoRes();
     mSlowScriptSecondHalf = false;
diff --git a/widget/EventMessageList.h b/widget/EventMessageList.h
index 7ff16c4841..03ae940dc8 100644
--- a/widget/EventMessageList.h
+++ b/widget/EventMessageList.h
@@ -118,7 +118,9 @@ NS_EVENT_MESSAGE(eImageAbort)
 NS_EVENT_MESSAGE(eLoadError)
 NS_EVENT_MESSAGE(eLoadEnd)
 NS_EVENT_MESSAGE(ePopState)
+NS_EVENT_MESSAGE(eRejectionHandled)
 NS_EVENT_MESSAGE(eStorage)
+NS_EVENT_MESSAGE(eUnhandledRejection)
 NS_EVENT_MESSAGE(eBeforeUnload)
 NS_EVENT_MESSAGE(eReadyStateChange)
 
diff --git a/xpcom/base/CycleCollectedJSContext.cpp b/xpcom/base/CycleCollectedJSContext.cpp
index a28b1ed3ba..d92283fb20 100644
--- a/xpcom/base/CycleCollectedJSContext.cpp
+++ b/xpcom/base/CycleCollectedJSContext.cpp
@@ -56,6 +56,7 @@
 #include "mozilla/CycleCollectedJSContext.h"
 #include <algorithm>
 #include "mozilla/ArrayUtils.h"
+#include "mozilla/AsyncEventDispatcher.h"
 #include "mozilla/AutoRestore.h"
 #include "mozilla/Move.h"
 #include "mozilla/MemoryReporting.h"
@@ -69,6 +70,8 @@
 #include "mozilla/dom/Promise.h"
 #include "mozilla/dom/PromiseBinding.h"
 #include "mozilla/dom/PromiseDebugging.h"
+#include "mozilla/dom/PromiseRejectionEvent.h"
+#include "mozilla/dom/PromiseRejectionEventBinding.h"
 #include "mozilla/dom/ScriptSettings.h"
 #include "jsprf.h"
 #include "js/Debug.h"
@@ -437,7 +440,7 @@ CycleCollectedJSContext::CycleCollectedJSContext()
   , mPrevGCNurseryCollectionCallback(nullptr)
   , mJSHolders(256)
   , mDoingStableStates(false)
-  , mDisableMicroTaskCheckpoint(false)
+  , mTargetedMicroTaskRecursionDepth(0)
   , mMicroTaskLevel(0)
   , mMicroTaskRecursionDepth(0)
   , mOutOfMemoryState(OOMState::OK)
@@ -458,8 +461,8 @@ CycleCollectedJSContext::~CycleCollectedJSContext()
   MOZ_ASSERT(!mDeferredFinalizerTable.Count());
 
   // Last chance to process any events.
-  ProcessMetastableStateQueue(mBaseRecursionDepth);
-  MOZ_ASSERT(mMetastableStateEvents.IsEmpty());
+  CleanupIDBTransactions(mBaseRecursionDepth);
+  MOZ_ASSERT(mPendingIDBTransactions.IsEmpty());
 
   ProcessStableStateQueue();
   MOZ_ASSERT(mStableStateEvents.IsEmpty());
@@ -467,8 +470,8 @@ CycleCollectedJSContext::~CycleCollectedJSContext()
   // Clear mPendingException first, since it might be cycle collected.
   mPendingException = nullptr;
 
-  MOZ_ASSERT(mDebuggerPromiseMicroTaskQueue.empty());
-  MOZ_ASSERT(mPromiseMicroTaskQueue.empty());
+  MOZ_ASSERT(mDebuggerMicroTaskQueue.empty());
+  MOZ_ASSERT(mPendingMicroTaskRunnables.empty());
 
   mUncaughtRejections.reset();
   mConsumedRejections.reset();
@@ -921,7 +924,7 @@ CycleCollectedJSContext::LargeAllocationFailureCallback(void* aData)
   self->OnLargeAllocationFailure();
 }
 
-class PromiseJobRunnable final : public Runnable
+class PromiseJobRunnable final : public MicroTaskRunnable
 {
 public:
   PromiseJobRunnable(JS::HandleObject aCallback, JS::HandleObject aAllocationSite,
@@ -935,14 +938,20 @@ public:
   }
 
 protected:
-  NS_IMETHOD
-  Run() override
+  virtual void Run(AutoSlowOperation& aAso) override
   {
     nsIGlobalObject* global = xpc::NativeGlobal(mCallback->CallbackPreserveColor());
     if (global && !global->IsDying()) {
       mCallback->Call("promise callback");
+      aAso.CheckForInterrupt();
     }
-    return NS_OK;
+  }
+
+  virtual bool Suppressed() override
+  {
+    nsIGlobalObject* global =
+      xpc::NativeGlobal(mCallback->CallbackPreserveColor());
+    return global && global->IsInSyncOperation();
   }
 
 private:
@@ -976,7 +985,7 @@ CycleCollectedJSContext::EnqueuePromiseJobCallback(JSContext* aCx,
   if (aIncumbentGlobal) {
     global = xpc::NativeGlobal(aIncumbentGlobal);
   }
-  nsCOMPtr<nsIRunnable> runnable = new PromiseJobRunnable(aJob, aAllocationSite, global);
+  RefPtr<MicroTaskRunnable> runnable = new PromiseJobRunnable(aJob, aAllocationSite, global);
   self->DispatchToMicroTask(runnable.forget());
   return true;
 }
@@ -988,15 +997,65 @@ CycleCollectedJSContext::PromiseRejectionTrackerCallback(JSContext* aCx,
                                                          PromiseRejectionHandlingState state,
                                                          void* aData)
 {
-#ifdef DEBUG
   CycleCollectedJSContext* self = static_cast<CycleCollectedJSContext*>(aData);
-#endif // DEBUG
+
   MOZ_ASSERT(aCx == self->Context());
   MOZ_ASSERT(Get() == self);
 
+  // TODO: Bug 1549351 - Promise rejection event should not be sent for
+  // cross-origin scripts
+
+
+  // See HTML 8.1.6.3 HostPromiseRejectionTracker
+
+  PromiseArray& aboutToBeNotified = self->mAboutToBeNotifiedRejectedPromises;
+  PromiseHashtable& outstanding = self->mOutstandingRejections;
+  uint64_t promiseID = JS::GetPromiseID(aPromise);
+
   if (state == PromiseRejectionHandlingState::Unhandled) {
-    PromiseDebugging::AddUncaughtRejection(aPromise);
+    // Step 5.
+    // Unhandled Promises first go to the about-to-be-notified queue, processed
+    // after Microtasks as NotifyUnhandledRejections
+    RefPtr<Promise> promise = Promise::CreateFromExisting(xpc::NativeGlobal(aPromise), aPromise);
+    aboutToBeNotified.AppendElement(promise);
   } else {
+    // Unhandled-then-handled Promises (ie.e Promises where the unhandledrejection event
+    // handler attached a handler) may fire the rejectionhandled event.
+
+    // Step 6.1.
+    for (size_t i = 0; i < aboutToBeNotified.Length(); i++) {
+      if (aboutToBeNotified[i] &&
+          aboutToBeNotified[i]->PromiseObj() == aPromise) {
+        // To avoid large amounts of memmoves, we don't shrink the vector
+        // here. Instead, we filter out nullptrs when iterating over the
+        // vector later.
+        aboutToBeNotified[i] = nullptr;
+        MOZ_ASSERT(!outstanding.Get(promiseID, nullptr));
+        return;
+      }
+    }
+    // Step 6.2. and 6.3.
+    RefPtr<Promise> promise;
+    if (outstanding.Remove(promiseID, getter_AddRefs(promise))) {
+      // Step 6.4.
+      nsIGlobalObject* global = xpc::NativeGlobal(aPromise);
+      if (nsCOMPtr<EventTarget> owner = do_QueryInterface(global)) {
+        // Step 6.5.
+        // The spec says to do this in a global task, but we're just queuing an event,
+        // which will execute in the next event loop either way.
+        PromiseRejectionEventInit init;
+        init.mPromise = Promise::CreateFromExisting(global, aPromise);
+        init.mReason = JS::GetPromiseResult(aPromise);
+
+        RefPtr<PromiseRejectionEvent> event = PromiseRejectionEvent::Constructor(owner,
+                                                  NS_LITERAL_STRING("rejectionhandled"), init);
+
+        RefPtr<AsyncEventDispatcher> asyncDispatcher = new AsyncEventDispatcher(owner, event);
+        asyncDispatcher->PostDOMEvent();
+      }
+    }
+
+    // Finally, notifiy debug observers
     PromiseDebugging::AddConsumedRejection(aPromise);
   }
 }
@@ -1175,18 +1234,18 @@ CycleCollectedJSContext::SetPendingException(nsIException* aException)
   mPendingException = aException;
 }
 
-std::queue<nsCOMPtr<nsIRunnable>>&
-CycleCollectedJSContext::GetPromiseMicroTaskQueue()
+std::queue<RefPtr<MicroTaskRunnable>>&
+CycleCollectedJSContext::GetMicroTaskQueue()
 {
   MOZ_ASSERT(mJSContext);
-  return mPromiseMicroTaskQueue;
+  return mPendingMicroTaskRunnables;
 }
 
-std::queue<nsCOMPtr<nsIRunnable>>&
-CycleCollectedJSContext::GetDebuggerPromiseMicroTaskQueue()
+std::queue<RefPtr<MicroTaskRunnable>>&
+CycleCollectedJSContext::GetDebuggerMicroTaskQueue()
 {
   MOZ_ASSERT(mJSContext);
-  return mDebuggerPromiseMicroTaskQueue;
+  return mDebuggerMicroTaskQueue;
 }
 
 nsCycleCollectionParticipant*
@@ -1345,24 +1404,24 @@ CycleCollectedJSContext::ProcessStableStateQueue()
 }
 
 void
-CycleCollectedJSContext::ProcessMetastableStateQueue(uint32_t aRecursionDepth)
+CycleCollectedJSContext::CleanupIDBTransactions(uint32_t aRecursionDepth)
 {
   MOZ_ASSERT(mJSContext);
   MOZ_RELEASE_ASSERT(!mDoingStableStates);
   mDoingStableStates = true;
 
-  nsTArray<RunInMetastableStateData> localQueue = Move(mMetastableStateEvents);
+  nsTArray<PendingIDBTransactionData> localQueue = Move(mPendingIDBTransactions);
 
   for (uint32_t i = 0; i < localQueue.Length(); ++i)
   {
-    RunInMetastableStateData& data = localQueue[i];
+    PendingIDBTransactionData& data = localQueue[i];
     if (data.mRecursionDepth != aRecursionDepth) {
       continue;
     }
 
     {
-      nsCOMPtr<nsIRunnable> runnable = data.mRunnable.forget();
-      runnable->Run();
+      nsCOMPtr<nsIRunnable> transaction = data.mTransaction.forget();
+      transaction->Run();
     }
 
     localQueue.RemoveElementAt(i--);
@@ -1370,11 +1429,27 @@ CycleCollectedJSContext::ProcessMetastableStateQueue(uint32_t aRecursionDepth)
 
   // If the queue has events in it now, they were added from something we called,
   // so they belong at the end of the queue.
-  localQueue.AppendElements(mMetastableStateEvents);
-  localQueue.SwapElements(mMetastableStateEvents);
+  localQueue.AppendElements(mPendingIDBTransactions);
+  localQueue.SwapElements(mPendingIDBTransactions);
   mDoingStableStates = false;
 }
 
+void
+CycleCollectedJSContext::BeforeProcessTask(bool aMightBlock)
+{
+  // If ProcessNextEvent was called during a microtask callback, we
+  // must process any pending microtasks before blocking in the event loop,
+  // otherwise we may deadlock until an event enters the queue later.
+  if (aMightBlock && PerformMicroTaskCheckPoint()) {
+    // If any microtask was processed, we post a dummy event in order to
+    // force the ProcessNextEvent call not to block.  This is required
+    // to support nested event loops implemented using a pattern like
+    // "while (condition) thread.processNextEvent(true)", in case the
+    // condition is triggered here by a Promise "then" callback.
+    NS_DispatchToMainThread(new Runnable());
+  }
+}
+
 void
 CycleCollectedJSContext::AfterProcessTask(uint32_t aRecursionDepth)
 {
@@ -1382,39 +1457,27 @@ CycleCollectedJSContext::AfterProcessTask(uint32_t aRecursionDepth)
 
   // See HTML 6.1.4.2 Processing model
 
-  // Execute any events that were waiting for a microtask to complete.
-  // This is not (yet) in the spec.
-  ProcessMetastableStateQueue(aRecursionDepth);
-
   // Step 4.1: Execute microtasks.
-  if (!mDisableMicroTaskCheckpoint) {
-    PerformMicroTaskCheckPoint();
-    if (NS_IsMainThread()) {
-      Promise::PerformMicroTaskCheckpoint();
-    } else {
-      Promise::PerformWorkerMicroTaskCheckpoint();
-    }
-  }
+  PerformMicroTaskCheckPoint();
 
   // Step 4.2 Execute any events that were waiting for a stable state.
   ProcessStableStateQueue();
 }
 
 void
-CycleCollectedJSContext::AfterProcessMicrotask()
+CycleCollectedJSContext::AfterProcessMicrotasks()
 {
   MOZ_ASSERT(mJSContext);
-  AfterProcessMicrotask(RecursionDepth());
-}
-
-void
-CycleCollectedJSContext::AfterProcessMicrotask(uint32_t aRecursionDepth)
-{
-  MOZ_ASSERT(mJSContext);
-
-  // Between microtasks, execute any events that were waiting for a microtask
-  // to complete.
-  ProcessMetastableStateQueue(aRecursionDepth);
+  // Notify unhandled promise rejections:
+  // https://html.spec.whatwg.org/multipage/webappapis.html#notify-about-rejected-promises
+  if (mAboutToBeNotifiedRejectedPromises.Length()) {
+    RefPtr<NotifyUnhandledRejections> runnable = new NotifyUnhandledRejections(
+        this, std::move(mAboutToBeNotifiedRejectedPromises));
+    NS_DispatchToCurrentThread(runnable);
+  }
+  // Cleanup Indexed Database transactions:
+  // https://html.spec.whatwg.org/multipage/webappapis.html#perform-a-microtask-checkpoint
+  CleanupIDBTransactions(RecursionDepth());
 }
 
 uint32_t
@@ -1431,12 +1494,12 @@ CycleCollectedJSContext::RunInStableState(already_AddRefed<nsIRunnable>&& aRunna
 }
 
 void
-CycleCollectedJSContext::RunInMetastableState(already_AddRefed<nsIRunnable>&& aRunnable)
+CycleCollectedJSContext::AddPendingIDBTransaction(already_AddRefed<nsIRunnable>&& aTransaction)
 {
   MOZ_ASSERT(mJSContext);
 
-  RunInMetastableStateData data;
-  data.mRunnable = aRunnable;
+  PendingIDBTransactionData data;
+  data.mTransaction = aTransaction;
 
   MOZ_ASSERT(mOwningThread);
   data.mRecursionDepth = RecursionDepth();
@@ -1453,7 +1516,7 @@ CycleCollectedJSContext::RunInMetastableState(already_AddRefed<nsIRunnable>&& aR
   }
 #endif
 
-  mMetastableStateEvents.AppendElement(Move(data));
+  mPendingIDBTransactions.AppendElement(Move(data));
 }
 
 IncrementalFinalizeRunnable::IncrementalFinalizeRunnable(CycleCollectedJSContext* aCx,
@@ -1659,14 +1722,14 @@ CycleCollectedJSContext::PrepareWaitingZonesForGC()
 }
 
 void
-CycleCollectedJSContext::DispatchToMicroTask(already_AddRefed<nsIRunnable> aRunnable)
+CycleCollectedJSContext::DispatchToMicroTask(already_AddRefed<MicroTaskRunnable> aRunnable)
 {
-  RefPtr<nsIRunnable> runnable(aRunnable);
+  RefPtr<MicroTaskRunnable> runnable(aRunnable);
 
   MOZ_ASSERT(NS_IsMainThread());
   MOZ_ASSERT(runnable);
 
-  mPromiseMicroTaskQueue.push(runnable.forget());
+  mPendingMicroTaskRunnables.push(runnable.forget());
 }
 
 class AsyncMutationHandler final : public mozilla::Runnable
@@ -1682,41 +1745,61 @@ public:
   }
 };
 
-void
+bool
 CycleCollectedJSContext::PerformMicroTaskCheckPoint()
 {
-  if (mPendingMicroTaskRunnables.empty()) {
+  if (mPendingMicroTaskRunnables.empty() && mDebuggerMicroTaskQueue.empty()) {
+    AfterProcessMicrotasks();
     // Nothing to do, return early.
-    return;
+    return false;
   }
 
   uint32_t currentDepth = RecursionDepth();
   if (mMicroTaskRecursionDepth >= currentDepth) {
     // We are already executing microtasks for the current recursion depth.
-    return;
+    return false;
+  }
+
+  if (mTargetedMicroTaskRecursionDepth != 0 &&
+      mTargetedMicroTaskRecursionDepth != currentDepth) {
+    return false;
   }
 
   if (NS_IsMainThread() && !nsContentUtils::IsSafeToRunScript()) {
     // Special case for main thread where DOM mutations may happen when
     // it is not safe to run scripts.
     nsContentUtils::AddScriptRunner(new AsyncMutationHandler());
-    return;
+    return false;
   }
 
   mozilla::AutoRestore<uint32_t> restore(mMicroTaskRecursionDepth);
   MOZ_ASSERT(currentDepth > 0);
   mMicroTaskRecursionDepth = currentDepth;
 
+  bool didProcess = false;
   AutoSlowOperation aso;
 
   std::queue<RefPtr<MicroTaskRunnable>> suppressed;
-  while (!mPendingMicroTaskRunnables.empty()) {
-    RefPtr<MicroTaskRunnable> runnable =
-      mPendingMicroTaskRunnables.front().forget();
-    mPendingMicroTaskRunnables.pop();
+  for (;;) {
+    RefPtr<MicroTaskRunnable> runnable;
+    if (!mDebuggerMicroTaskQueue.empty()) {
+      runnable = mDebuggerMicroTaskQueue.front().forget();
+      mDebuggerMicroTaskQueue.pop();
+    } else if (!mPendingMicroTaskRunnables.empty()) {
+      runnable = mPendingMicroTaskRunnables.front().forget();
+      mPendingMicroTaskRunnables.pop();
+    } else {
+      break;
+    }
+
     if (runnable->Suppressed()) {
+      // Microtasks in worker shall never be suppressed.
+      // Otherwise, mPendingMicroTaskRunnables will be replaced later with
+      // all suppressed tasks in mDebuggerMicroTaskQueue unexpectedly.
+      MOZ_ASSERT(NS_IsMainThread());
       suppressed.push(runnable);
     } else {
+      didProcess = true;
       runnable->Run(aso);
     }
   }
@@ -1726,13 +1809,105 @@ CycleCollectedJSContext::PerformMicroTaskCheckPoint()
   // for some time, but no longer than spinning the event loop nestedly
   // (sync XHR, alert, etc.)
   mPendingMicroTaskRunnables.swap(suppressed);
+
+  AfterProcessMicrotasks();
+
+  return didProcess;
 }
 
 void
-CycleCollectedJSContext::DispatchMicroTaskRunnable(
-  already_AddRefed<MicroTaskRunnable> aRunnable)
+CycleCollectedJSContext::PerformDebuggerMicroTaskCheckpoint()
+ {
+  // Don't do normal microtask handling checks here, since whoever is calling
+  // this method is supposed to know what they are doing.
+
+  AutoSlowOperation aso;
+  for (;;) {
+    // For a debugger microtask checkpoint, we always use the debugger microtask
+    // queue.
+    std::queue<RefPtr<MicroTaskRunnable>>* microtaskQueue =
+      &GetDebuggerMicroTaskQueue();
+
+    if (microtaskQueue->empty()) {
+      break;
+    }
+
+    RefPtr<MicroTaskRunnable> runnable = microtaskQueue->front().forget();
+    MOZ_ASSERT(runnable);
+
+    // This function can re-enter, so we remove the element before calling.
+    microtaskQueue->pop();
+    runnable->Run(aso);
+  }
+
+  AfterProcessMicrotasks();
+}
+
+NS_IMETHODIMP
+CycleCollectedJSContext::NotifyUnhandledRejections::Run()
 {
-  mPendingMicroTaskRunnables.push(aRunnable);
+  // See HTML 8.1.4.7 Unhandled promise rejections
+  // with 'mUnhandledRejections' == copy of 'about-to-be-notified rejected'
+
+  for (size_t i = 0; i < mUnhandledRejections.Length(); ++i) {
+    // Step 5.1.
+    RefPtr<Promise>& promise = mUnhandledRejections[i];
+    if (!promise)
+      continue;
+
+    JS::RootedObject promiseObj(mCx->RootingCx(), promise->PromiseObj());
+    MOZ_ASSERT(JS::IsPromiseObject(promiseObj));
+
+    // Step 5.1.1.
+    if (JS::GetPromiseIsHandled(promiseObj))
+      continue;
+
+    uint64_t promiseID = JS::GetPromiseID(promiseObj);
+
+    // Step 5.1.2.
+    bool raiseError = true;
+    if (nsCOMPtr<EventTarget> target =
+            do_QueryInterface(promise->GetParentObject())) {
+      PromiseRejectionEventInit init;
+      init.mPromise = promise;
+      init.mReason = JS::GetPromiseResult(promiseObj);
+      init.mCancelable = true;
+
+      RefPtr<PromiseRejectionEvent> event =
+          PromiseRejectionEvent::Constructor(
+              target, NS_LITERAL_STRING("unhandledrejection"), init);
+
+      // Step 5.1.4. (reordered)
+      // In contrast to the letter of the spec, this must be done before
+      // the event handler is called or the PromiseRejectionTrackerCallback
+      // will not see the correct state.
+      PromiseDebugging::AddUncaughtRejection(promiseObj);
+      target->DispatchEvent(event, &raiseError);
+    }
+
+    // Step 5.1.3. (implied)
+    // If the Promise became handled, PromiseRejectionTrackerCallback will have marked it.
+    // If not, keep it in the "uncaught" list for PromiseDebugging, and set the flag if
+    // preventDefault indicated we don't want it reported to console.
+    if (!JS::GetPromiseIsHandled(promiseObj) && !raiseError) {
+      JS::MarkPromiseRejectionReported(promiseObj);
+    }
+  }
+  return NS_OK;
+}
+
+nsresult
+CycleCollectedJSContext::NotifyUnhandledRejections::Cancel()
+{
+  for (size_t i = 0; i < mUnhandledRejections.Length(); ++i) {
+    RefPtr<Promise>& promise = mUnhandledRejections[i];
+    if (!promise)
+      continue;
+
+    JS::RootedObject promiseObj(mCx->RootingCx(), promise->PromiseObj());
+    mCx->mOutstandingRejections.Remove(JS::GetPromiseID(promiseObj));
+  }
+  return NS_OK;
 }
 
 void
diff --git a/xpcom/base/CycleCollectedJSContext.h b/xpcom/base/CycleCollectedJSContext.h
index b9fc8e6045..4e387bae1a 100644
--- a/xpcom/base/CycleCollectedJSContext.h
+++ b/xpcom/base/CycleCollectedJSContext.h
@@ -12,6 +12,7 @@
 #include "mozilla/mozalloc.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/SegmentedVector.h"
+#include "mozilla/dom/Promise.h"
 #include "jsapi.h"
 #include "jsfriendapi.h"
 
@@ -170,9 +171,6 @@ protected:
   virtual void CustomOutOfMemoryCallback() {}
   virtual void CustomLargeAllocationFailureCallback() {}
 
-  std::queue<nsCOMPtr<nsIRunnable>> mPromiseMicroTaskQueue;
-  std::queue<nsCOMPtr<nsIRunnable>> mDebuggerPromiseMicroTaskQueue;
-
 private:
   void
   DescribeGCThing(bool aIsMarked, JS::GCCellPtr aThing,
@@ -243,11 +241,11 @@ private:
   virtual void TraceNativeBlackRoots(JSTracer* aTracer) { };
   void TraceNativeGrayRoots(JSTracer* aTracer);
 
-  void AfterProcessMicrotask(uint32_t aRecursionDepth);
+  void AfterProcessMicrotasks();
 public:
   void ProcessStableStateQueue();
 private:
-  void ProcessMetastableStateQueue(uint32_t aRecursionDepth);
+  void CleanupIDBTransactions(uint32_t aRecursionDepth);
 
 public:
   enum DeferredFinalizeType {
@@ -306,8 +304,8 @@ public:
   already_AddRefed<nsIException> GetPendingException() const;
   void SetPendingException(nsIException* aException);
 
-  std::queue<nsCOMPtr<nsIRunnable>>& GetPromiseMicroTaskQueue();
-  std::queue<nsCOMPtr<nsIRunnable>>& GetDebuggerPromiseMicroTaskQueue();
+  std::queue<RefPtr<MicroTaskRunnable>>& GetMicroTaskQueue();
+  std::queue<RefPtr<MicroTaskRunnable>>& GetDebuggerMicroTaskQueue();
 
   nsCycleCollectionParticipant* GCThingParticipant();
   nsCycleCollectionParticipant* ZoneParticipant();
@@ -346,53 +344,25 @@ public:
     return JS::RootingContext::get(mJSContext);
   }
 
-  bool MicroTaskCheckpointDisabled() const
+  void SetTargetedMicroTaskRecursionDepth(uint32_t aDepth)
   {
-    return mDisableMicroTaskCheckpoint;
+    mTargetedMicroTaskRecursionDepth = aDepth;
   }
 
-  void DisableMicroTaskCheckpoint(bool aDisable)
-  {
-    mDisableMicroTaskCheckpoint = aDisable;
-  }
-
-  class MOZ_RAII AutoDisableMicroTaskCheckpoint
-  {
-    public:
-    AutoDisableMicroTaskCheckpoint()
-    : mCCJSCX(CycleCollectedJSContext::Get())
-    {
-      mOldValue = mCCJSCX->MicroTaskCheckpointDisabled();
-      mCCJSCX->DisableMicroTaskCheckpoint(true);
-    }
-
-    ~AutoDisableMicroTaskCheckpoint()
-    {
-      mCCJSCX->DisableMicroTaskCheckpoint(mOldValue);
-    }
-
-    CycleCollectedJSContext* mCCJSCX;
-    bool mOldValue;
-  };
-
 protected:
   JSContext* MaybeContext() const { return mJSContext; }
 
 public:
   // nsThread entrypoints
-  virtual void BeforeProcessTask(bool aMightBlock) { };
+  virtual void BeforeProcessTask(bool aMightBlock);
   virtual void AfterProcessTask(uint32_t aRecursionDepth);
 
-  // microtask processor entry point
-  void AfterProcessMicrotask();
-
   uint32_t RecursionDepth();
 
   // Run in stable state (call through nsContentUtils)
   void RunInStableState(already_AddRefed<nsIRunnable>&& aRunnable);
-  // This isn't in the spec at all yet, but this gets the behavior we want for IDB.
-  // Runs after the current microtask completes.
-  void RunInMetastableState(already_AddRefed<nsIRunnable>&& aRunnable);
+
+  void AddPendingIDBTransaction(already_AddRefed<nsIRunnable>&& aTransaction);
 
   // Get the current thread's CycleCollectedJSContext.  Returns null if there
   // isn't one.
@@ -411,7 +381,7 @@ public:
   void PrepareWaitingZonesForGC();
 
   // Queue an async microtask to the current main or worker thread.
-  virtual void DispatchToMicroTask(already_AddRefed<nsIRunnable> aRunnable);
+  virtual void DispatchToMicroTask(already_AddRefed<MicroTaskRunnable> aRunnable);
 
   // Call EnterMicroTask when you're entering JS execution.
   // Usually the best way to do this is to use nsAutoMicroTask.
@@ -442,9 +412,9 @@ public:
     mMicroTaskLevel = aLevel;
   }
 
-  void PerformMicroTaskCheckPoint();
+  bool PerformMicroTaskCheckPoint();
 
-  void DispatchMicroTaskRunnable(already_AddRefed<MicroTaskRunnable> aRunnable);
+  void PerformDebuggerMicroTaskCheckpoint();
 
   // Storage for watching rejected promises waiting for some client to
   // consume their rejection.
@@ -460,6 +430,12 @@ public:
   // in the last turn of the event loop.
   JS::PersistentRooted<JS::GCVector<JSObject*, 0, js::SystemAllocPolicy>> mConsumedRejections;
 
+  // This is for the "outstanding rejected promises weak set" in the spec,
+  // https://html.spec.whatwg.org/multipage/webappapis.html#outstanding-rejected-promises-weak-set
+  typedef nsRefPtrHashtable<nsUint64HashKey, dom::Promise> PromiseHashtable;
+  PromiseHashtable mOutstandingRejections;
+
+
   nsTArray<nsCOMPtr<nsISupports /* UncaughtRejectionObserver */ >> mUncaughtRejectionObservers;
 
 private:
@@ -483,24 +459,50 @@ private:
   nsCOMPtr<nsIException> mPendingException;
   nsThread* mOwningThread; // Manual refcounting to avoid include hell.
 
-  struct RunInMetastableStateData
+  struct PendingIDBTransactionData
   {
-    nsCOMPtr<nsIRunnable> mRunnable;
+    nsCOMPtr<nsIRunnable> mTransaction;
     uint32_t mRecursionDepth;
   };
 
   nsTArray<nsCOMPtr<nsIRunnable>> mStableStateEvents;
-  nsTArray<RunInMetastableStateData> mMetastableStateEvents;
+  nsTArray<PendingIDBTransactionData> mPendingIDBTransactions;
   uint32_t mBaseRecursionDepth;
   bool mDoingStableStates;
 
-  bool mDisableMicroTaskCheckpoint;
+  // If set to none 0, microtasks will be processed only when recursion depth
+  // is the set value.
+  uint32_t mTargetedMicroTaskRecursionDepth;
 
   uint32_t mMicroTaskLevel;
+
   std::queue<RefPtr<MicroTaskRunnable>> mPendingMicroTaskRunnables;
+  std::queue<RefPtr<MicroTaskRunnable>> mDebuggerMicroTaskQueue;
 
   uint32_t mMicroTaskRecursionDepth;
 
+  // This implements about-to-be-notified rejected promises list in the spec.
+  // https://html.spec.whatwg.org/multipage/webappapis.html#about-to-be-notified-rejected-promises-list
+  typedef nsTArray<RefPtr<dom::Promise>> PromiseArray;
+  PromiseArray mAboutToBeNotifiedRejectedPromises;
+
+  class NotifyUnhandledRejections final : public CancelableRunnable {
+   public:
+    NotifyUnhandledRejections(CycleCollectedJSContext* aCx,
+                              PromiseArray&& aPromises)
+        : CancelableRunnable(),
+          mCx(aCx),
+          mUnhandledRejections(std::move(aPromises)) {}
+
+    NS_IMETHOD Run() final;
+
+    nsresult Cancel() final;
+
+   private:
+    CycleCollectedJSContext* mCx;
+    PromiseArray mUnhandledRejections;
+  };
+
   OOMState mOutOfMemoryState;
   OOMState mLargeAllocationFailureState;