diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index 1be4e39113..bb143a4998 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -4949,6 +4949,13 @@ 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)
 {
diff --git a/js/src/jsapi.h b/js/src/jsapi.h
index 444b9e2ed2..97c085b330 100644
--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -4611,6 +4611,15 @@ 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 a js::SavedFrame linked list of the stack that lead to the given
  * Promise's allocation.
diff --git a/xpcom/base/CycleCollectedJSContext.cpp b/xpcom/base/CycleCollectedJSContext.cpp
index bbf8bca623..9d033c33b8 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"
@@ -994,16 +997,53 @@ 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
+
+  PromiseArray& aboutToBeNotified = self->mAboutToBeNotifiedRejectedPromises;
+  PromiseHashtable& unhandled = self->mPendingUnhandledRejections;
+  uint64_t promiseID = JS::GetPromiseID(aPromise);
+
   if (state == PromiseRejectionHandlingState::Unhandled) {
     PromiseDebugging::AddUncaughtRejection(aPromise);
+    RefPtr<Promise> promise = Promise::CreateFromExisting(xpc::NativeGlobal(aPromise), aPromise);
+    aboutToBeNotified.AppendElement(promise);
+    unhandled.Put(promiseID, promise);
   } else {
     PromiseDebugging::AddConsumedRejection(aPromise);
+    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(unhandled.Get(promiseID, nullptr));
+        unhandled.Remove(promiseID);
+        return;
+      }
+    }
+    RefPtr<Promise> promise;
+    unhandled.Remove(promiseID, getter_AddRefs(promise));
+    if (!promise) {
+      nsIGlobalObject* global = xpc::NativeGlobal(aPromise);
+      if (nsCOMPtr<EventTarget> owner = do_QueryInterface(global)) {
+        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();
+      }
+    }
   }
 }
 
@@ -1415,6 +1455,13 @@ void
 CycleCollectedJSContext::AfterProcessMicrotasks()
 {
   MOZ_ASSERT(mJSContext);
+  // 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());
@@ -1783,6 +1830,64 @@ CycleCollectedJSContext::PerformDebuggerMicroTaskCheckpoint()
   AfterProcessMicrotasks();
 }
 
+NS_IMETHODIMP
+CycleCollectedJSContext::NotifyUnhandledRejections::Run()
+{
+  for (size_t i = 0; i < mUnhandledRejections.Length(); ++i) {
+    RefPtr<Promise>& promise = mUnhandledRejections[i];
+    if (!promise)
+      continue;
+
+    JS::RootedObject promiseObj(mCx->RootingCx(), promise->PromiseObj());
+    MOZ_ASSERT(JS::IsPromiseObject(promiseObj));
+
+    // Only fire unhandledrejection if the promise is still not handled;
+    uint64_t promiseID = JS::GetPromiseID(promiseObj);
+    if (!JS::GetPromiseIsHandled(promiseObj)) {
+      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);
+        // We don't use the result of dispatching event here to check whether to
+        // report the Promise to console.
+        bool dummy = true;
+        target->DispatchEvent(event, &dummy);
+      }
+    }
+
+    if (!JS::GetPromiseIsHandled(promiseObj)) {
+      MOZ_ASSERT(mCx->mPendingUnhandledRejections.Get(promiseID, nullptr));
+      mCx->mPendingUnhandledRejections.Remove(promiseID);
+    }
+
+    // If a rejected promise is being handled in "unhandledrejection" event
+    // handler, it should be removed from the table in
+    // PromiseRejectionTrackerCallback.
+    MOZ_ASSERT(!mCx->mPendingUnhandledRejections.Get(promiseID, nullptr));
+  }
+  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->mPendingUnhandledRejections.Remove(JS::GetPromiseID(promiseObj));
+  }
+  return NS_OK;
+}
+
 void
 CycleCollectedJSContext::EnvironmentPreparer::invoke(JS::HandleObject scope,
                                                      js::ScriptEnvironmentPreparer::Closure& closure)
diff --git a/xpcom/base/CycleCollectedJSContext.h b/xpcom/base/CycleCollectedJSContext.h
index d106d54ae9..63669bffa7 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"
 
@@ -474,6 +475,47 @@ private:
 
   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;
+
+  // 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
+  // We use different data structure and opposite logic here to achieve the same
+  // effect. Basically this is used for tracking the rejected promise that does
+  // NOT need firing a rejectionhandled event. We will check the table to see if
+  // firing rejectionhandled event is required when a rejected promise is being
+  // handled.
+  //
+  // The rejected promise will be stored in the table if
+  // - it is unhandled, and
+  // - The unhandledrejection is not yet fired.
+  //
+  // And be removed when
+  // - it is handled, or
+  // - A unhandledrejection is fired and it isn't being handled in event
+  // handler.
+  typedef nsRefPtrHashtable<nsUint64HashKey, dom::Promise> PromiseHashtable;
+  PromiseHashtable mPendingUnhandledRejections;
+
+  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;