--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -4057,18 +4057,19 @@ PresShell::HandlePostedReflowCallbacks(b
    if (shouldFlush && !mIsDestroying) {
      FlushPendingNotifications(flushType);
    }
 }
 
 bool
 nsIPresShell::IsSafeToFlush() const
 {
-  // Not safe if we are reflowing or in the middle of frame construction
-  if (mIsReflowing || mChangeNestCount) {
+  // Not safe if we are getting torn down, reflowing, or in the middle of frame
+  // construction.
+  if (mIsReflowing || mChangeNestCount || mIsDestroying) {
     return false;
   }
 
     // Not safe if we are painting
   if (nsViewManager* viewManager = GetViewManager()) {
     bool isPainting = false;
     viewManager->IsPainting(isPainting);
     if (isPainting) {
@@ -4097,18 +4098,16 @@ PresShell::DoFlushPendingNotifications(m
   /**
    * VERY IMPORTANT: If you add some sort of new flushing to this
    * method, make sure to add the relevant SetNeedLayoutFlush or
    * SetNeedStyleFlush calls on the shell.
    */
   FlushType flushType = aFlush.mFlushType;
 
   MOZ_ASSERT(NeedFlush(flushType), "Why did we get called?");
-  MOZ_DIAGNOSTIC_ASSERT(mDocument->HasShellOrBFCacheEntry());
-  MOZ_DIAGNOSTIC_ASSERT(mDocument->GetShell() == this);
 
 #ifdef MOZ_GECKO_PROFILER
   static const EnumeratedArray<FlushType,
                                FlushType::Count,
                                const char*> flushTypeNames = {
     "",
     "Event",
     "Content",
@@ -4148,22 +4147,26 @@ PresShell::DoFlushPendingNotifications(m
   // If layout could possibly trigger scripts, then it's only safe to flush if
   // it's safe to run script.
   bool hasHadScriptObject;
   if (mDocument->GetScriptHandlingObject(hasHadScriptObject) ||
       hasHadScriptObject) {
     isSafeToFlush = isSafeToFlush && nsContentUtils::IsSafeToRunScript();
   }
 
-  NS_ASSERTION(!isSafeToFlush || mViewManager, "Must have view manager");
+  MOZ_DIAGNOSTIC_ASSERT(!mIsDestroying || !isSafeToFlush);
+  MOZ_DIAGNOSTIC_ASSERT(mIsDestroying || mViewManager);
+  MOZ_DIAGNOSTIC_ASSERT(mIsDestroying || mDocument->HasShellOrBFCacheEntry());
+  MOZ_DIAGNOSTIC_ASSERT(mIsDestroying || mDocument->GetShell() == this);
+
   // Make sure the view manager stays alive.
   RefPtr<nsViewManager> viewManager = mViewManager;
   bool didStyleFlush = false;
   bool didLayoutFlush = false;
-  if (isSafeToFlush && viewManager) {
+  if (isSafeToFlush) {
     // Record that we are in a flush, so that our optimization in
     // nsDocument::FlushPendingNotifications doesn't skip any re-entrant
     // calls to us.  Otherwise, we might miss some needed flushes, since
     // we clear mNeedStyleFlush / mNeedLayoutFlush here at the top of
     // the function but we might not have done the work yet.
     AutoRestore<bool> guard(mInFlush);
     mInFlush = true;