diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index 2ff5273d93..7ad4be98da 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -893,6 +893,8 @@ nsDocShell::~nsDocShell()
 nsresult
 nsDocShell::Init()
 {
+  MOZ_ASSERT(!mIsBeingDestroyed);
+
   nsresult rv = nsDocLoader::Init();
   NS_ENSURE_SUCCESS(rv, rv);
 
@@ -1921,6 +1923,8 @@ bool
 nsDocShell::SetCurrentURI(nsIURI* aURI, nsIRequest* aRequest,
                           bool aFireOnLocationChange, uint32_t aLocationFlags)
 {
+  MOZ_ASSERT(!mIsBeingDestroyed);
+
   if (gDocShellLeakLog && MOZ_LOG_TEST(gDocShellLeakLog, LogLevel::Debug)) {
     PR_LogPrint("DOCSHELL %p SetCurrentURI %s\n",
                 this, aURI ? aURI->GetSpecOrDefault().get() : "");
@@ -2181,6 +2185,8 @@ nsDocShell::SetUsePrivateBrowsing(bool aUsePrivateBrowsing)
 NS_IMETHODIMP
 nsDocShell::SetPrivateBrowsing(bool aUsePrivateBrowsing)
 {
+  MOZ_ASSERT(!mIsBeingDestroyed);
+
   bool changed = aUsePrivateBrowsing != (mPrivateBrowsingId > 0);
   if (changed) {
     mPrivateBrowsingId = aUsePrivateBrowsing ? 1 : 0;
@@ -2251,6 +2257,8 @@ nsDocShell::SetRemoteTabs(bool aUseRemoteTabs)
 NS_IMETHODIMP
 nsDocShell::SetAffectPrivateSessionLifetime(bool aAffectLifetime)
 {
+  MOZ_ASSERT(!mIsBeingDestroyed);
+
   bool change = aAffectLifetime != mAffectPrivateSessionLifetime;
   if (change && UsePrivateBrowsing()) {
     AssertOriginAttributesMatchPrivateBrowsing();
@@ -2753,6 +2761,8 @@ nsDocShell::GetSecurityUI(nsISecureBrowserUI** aSecurityUI)
 NS_IMETHODIMP
 nsDocShell::SetSecurityUI(nsISecureBrowserUI* aSecurityUI)
 {
+  MOZ_ASSERT(!mIsBeingDestroyed);
+
   mSecurityUI = aSecurityUI;
   mSecurityUI->SetDocShell(this);
   return NS_OK;
@@ -3935,6 +3945,10 @@ PrintDocTree(nsIDocShellTreeItem* aParentNode)
 NS_IMETHODIMP
 nsDocShell::SetTreeOwner(nsIDocShellTreeOwner* aTreeOwner)
 {
+  if (mIsBeingDestroyed && aTreeOwner) {
+    return NS_ERROR_FAILURE;
+  }
+
 #ifdef DEBUG_DOCSHELL_FOCUS
   nsCOMPtr<nsIDocShellTreeItem> item(do_QueryInterface(aTreeOwner));
   if (item) {
@@ -5216,6 +5230,8 @@ nsDocShell::LoadErrorPage(nsIURI* aURI, const char16_t* aURL,
                           const char* aCSSClass,
                           nsIChannel* aFailedChannel)
 {
+  MOZ_ASSERT(!mIsBeingDestroyed);
+
 #if defined(DEBUG)
   if (MOZ_LOG_TEST(gDocShellLog, LogLevel::Debug)) {
     nsAutoCString chanName;
@@ -5571,6 +5587,8 @@ nsDocShell::SetSessionHistory(nsISHistory* aSessionHistory)
   // make sure that we are the root docshell and
   // set a handle to root docshell in SH.
 
+  MOZ_ASSERT(!mIsBeingDestroyed);
+
   nsCOMPtr<nsIDocShellTreeItem> root;
   /* Get the root docshell. If *this* is the root docshell
    * then save a handle to *this* in SH. SH needs it to do
@@ -5751,16 +5769,22 @@ nsDocShell::Create()
 NS_IMETHODIMP
 nsDocShell::Destroy()
 {
+  // XXX: We allow this function to be called just once.  If you are going to
+  // reset new variables in this function, please make sure the variables will
+  // never be re-initialized.  Adding assertions to check |mIsBeingDestroyed|
+  // in the setter functions for the variables would be enough.
+  if (mIsBeingDestroyed) {
+    return NS_ERROR_DOCSHELL_DYING;
+  }
+
   NS_ASSERTION(mItemType == typeContent || mItemType == typeChrome,
                "Unexpected item type in docshell");
 
-  if (!mIsBeingDestroyed) {
-    nsCOMPtr<nsIObserverService> serv = services::GetObserverService();
-    if (serv) {
-      const char* msg = mItemType == typeContent ?
-        NS_WEBNAVIGATION_DESTROY : NS_CHROME_WEBNAVIGATION_DESTROY;
-      serv->NotifyObservers(GetAsSupports(this), msg, nullptr);
-    }
+  nsCOMPtr<nsIObserverService> serv = services::GetObserverService();
+  if (serv) {
+    const char* msg = mItemType == typeContent ?
+      NS_WEBNAVIGATION_DESTROY : NS_CHROME_WEBNAVIGATION_DESTROY;
+    serv->NotifyObservers(GetAsSupports(this), msg, nullptr);
   }
 
   mIsBeingDestroyed = true;
@@ -6043,6 +6067,7 @@ nsDocShell::GetParentWidget(nsIWidget** aParentWidget)
 NS_IMETHODIMP
 nsDocShell::SetParentWidget(nsIWidget* aParentWidget)
 {
+  MOZ_ASSERT(!mIsBeingDestroyed);
   mParentWidget = aParentWidget;
 
   return NS_OK;
@@ -6313,6 +6338,8 @@ nsDocShell::SetOnePermittedSandboxedNavigator(nsIDocShell* aSandboxedNavigator)
     return NS_OK;
   }
 
+  MOZ_ASSERT(!mIsBeingDestroyed);
+
   mOnePermittedSandboxedNavigator = do_GetWeakReference(aSandboxedNavigator);
   NS_ASSERTION(mOnePermittedSandboxedNavigator,
                "One Permitted Sandboxed Navigator must support weak references.");
@@ -6673,6 +6700,8 @@ nsDocShell::RefreshURI(nsIURI* aURI,
                        bool aMetaRefresh,
                        nsIPrincipal* aPrincipal)
 {
+  MOZ_ASSERT(!mIsBeingDestroyed);
+
   NS_ENSURE_ARG(aURI);
 
   /* Check if Meta refresh/redirects are permitted. Some
@@ -8056,6 +8085,10 @@ nsDocShell::CreateAboutBlankContentViewer(nsIPrincipal* aPrincipal,
     // wrong information :-(
     //
     (void)FirePageHideNotification(!mSavingOldViewer);
+    // pagehide notification might destroy this docshell.
+    if (mIsBeingDestroyed) {
+      return NS_ERROR_DOCSHELL_DYING;
+    }
   }
 
   // Now make sure we don't think we're in the middle of firing unload after
@@ -8199,6 +8232,8 @@ nsDocShell::CanSavePresentation(uint32_t aLoadType,
 void
 nsDocShell::ReattachEditorToWindow(nsISHEntry* aSHEntry)
 {
+  MOZ_ASSERT(!mIsBeingDestroyed);
+
   NS_ASSERTION(!mEditorData,
                "Why reattach an editor when we already have one?");
   NS_ASSERTION(aSHEntry && aSHEntry->HasDetachedEditor(),
@@ -8236,6 +8271,9 @@ nsDocShell::DetachEditorFromWindow()
   if (NS_SUCCEEDED(res)) {
     // Make mOSHE hold the owning ref to the editor data.
     if (mOSHE) {
+      MOZ_ASSERT(!mIsBeingDestroyed || !mOSHE->HasDetachedEditor(),
+                 "We should not set the editor data again once after we "
+                 "detached the editor data during destroying this docshell");
       mOSHE->SetEditorData(mEditorData.forget());
     } else {
       mEditorData = nullptr;
@@ -8422,6 +8460,8 @@ nsDocShell::GetRestoringDocument(bool* aRestoring)
 nsresult
 nsDocShell::RestorePresentation(nsISHEntry* aSHEntry, bool* aRestoring)
 {
+  MOZ_ASSERT(!mIsBeingDestroyed);
+
   NS_ASSERTION(mLoadType & LOAD_CMD_HISTORY,
                "RestorePresentation should only be called for history loads");
 
@@ -8583,6 +8623,10 @@ nsDocShell::RestoreFromHistory()
 
   // Notify the old content viewer that it's being hidden.
   FirePageHideNotification(!mSavingOldViewer);
+  // pagehide notification might destroy this docshell.
+  if (mIsBeingDestroyed) {
+    return NS_ERROR_DOCSHELL_DYING;
+  }
 
   // If mLSHE was changed as a result of the pagehide event, then
   // something else was loaded.  Don't finish restoring.
@@ -9040,7 +9084,7 @@ nsDocShell::CreateContentViewer(const nsACString& aContentType,
 {
   *aContentHandler = nullptr;
 
-  if (!mTreeOwner) {
+  if (!mTreeOwner || mIsBeingDestroyed) {
     // If we don't have a tree owner, then we're in the process of being
     // destroyed. Rather than continue trying to load something, just give up.
     return NS_ERROR_DOCSHELL_DYING;
@@ -9085,6 +9129,11 @@ nsDocShell::CreateContentViewer(const nsACString& aContentType,
     aOpenedChannel->GetURI(getter_AddRefs(mLoadingURI));
   }
   FirePageHideNotification(!mSavingOldViewer);
+  if (mIsBeingDestroyed) {
+    // Force to stop the newly created orphaned viewer.
+    viewer->Stop();
+    return NS_ERROR_DOCSHELL_DYING;
+  }
   mLoadingURI = nullptr;
 
   // Set mFiredUnloadEvent = false so that the unload handler for the
@@ -9274,6 +9323,8 @@ nsDocShell::NewContentViewerObj(const nsACString& aContentType,
 nsresult
 nsDocShell::SetupNewViewer(nsIContentViewer* aNewViewer)
 {
+  MOZ_ASSERT(!mIsBeingDestroyed);
+
   //
   // Copy content viewer state from previous or parent content viewer.
   //
@@ -13466,6 +13517,8 @@ nsDocShell::EnsureScriptEnvironment()
 NS_IMETHODIMP
 nsDocShell::EnsureEditorData()
 {
+  MOZ_ASSERT(!mIsBeingDestroyed);
+
   bool openDocHasDetachedEditor = mOSHE && mOSHE->HasDetachedEditor();
   if (!mEditorData && !mIsBeingDestroyed && !openDocHasDetachedEditor) {
     // We shouldn't recreate the editor data if it already exists, or
@@ -13481,6 +13534,8 @@ nsDocShell::EnsureEditorData()
 nsresult
 nsDocShell::EnsureTransferableHookData()
 {
+  MOZ_ASSERT(!mIsBeingDestroyed);
+
   if (!mTransferableHookData) {
     mTransferableHookData = new nsTransferableHookData();
   }
@@ -14568,6 +14623,8 @@ nsDocShell::CanSetOriginAttributes()
 nsresult
 nsDocShell::SetOriginAttributes(const DocShellOriginAttributes& aAttrs)
 {
+  MOZ_ASSERT(!mIsBeingDestroyed);
+
   if (!CanSetOriginAttributes()) {
     return NS_ERROR_FAILURE;
   }
diff --git a/dom/base/nsContentSink.cpp b/dom/base/nsContentSink.cpp
index 0392fce500..d8760db70c 100644
--- a/dom/base/nsContentSink.cpp
+++ b/dom/base/nsContentSink.cpp
@@ -100,6 +100,7 @@ nsContentSink::nsContentSink()
   , mDeferredFlushTags(0)
   , mIsDocumentObserver(0)
   , mRunsToCompletion(0)
+  , mIsBlockingOnload(false)
   , mDeflectedCount(0)
   , mHasPendingEvent(false)
   , mCurrentParseEndTime(0)
@@ -1539,8 +1540,14 @@ nsContentSink::DropParserAndPerfHint(void)
     FavorPerformanceHint(true, 0);
   }
 
-  if (!mRunsToCompletion) {
+  // Call UnblockOnload only if mRunsToComletion is false and if
+  // we have already started loading because it's possible that this function
+  // is called (i.e. the parser is terminated) before we start loading due to
+  // destroying the window inside unload event callbacks for the previous
+  // document.
+  if (!mRunsToCompletion && mIsBlockingOnload) {
     mDocument->UnblockOnload(true);
+    mIsBlockingOnload = false;
   }
 }
 
@@ -1595,6 +1602,7 @@ nsContentSink::WillBuildModelImpl()
 {
   if (!mRunsToCompletion) {
     mDocument->BlockOnload();
+    mIsBlockingOnload = true;
 
     mBeginLoadTime = PR_IntervalToMicroseconds(PR_IntervalNow());
   }
diff --git a/dom/base/nsContentSink.h b/dom/base/nsContentSink.h
index 43a873dd6f..4140a3f19f 100644
--- a/dom/base/nsContentSink.h
+++ b/dom/base/nsContentSink.h
@@ -304,6 +304,8 @@ protected:
   // True if this is parser is a fragment parser or an HTML DOMParser.
   // XML DOMParser leaves this to false for now!
   uint8_t mRunsToCompletion : 1;
+  // True if we are blocking load event.
+  bool mIsBlockingOnload : 1;
   
   //
   // -- Can interrupt parsing members --