--- a/dom/media/MediaShutdownManager.cpp
+++ b/dom/media/MediaShutdownManager.cpp
@@ -86,34 +86,39 @@ MediaShutdownManager::RemoveBlocker()
   MOZ_ASSERT(mDecoders.Count() == 0);
   GetShutdownBarrier()->RemoveBlocker(this);
   // Clear our singleton reference. This will probably delete
   // this instance, so don't deref |this| clearing sInstance.
   sInstance = nullptr;
   DECODER_LOG(LogLevel::Debug, ("MediaShutdownManager::BlockShutdown() end."));
 }
 
-void
+nsresult
 MediaShutdownManager::Register(MediaDecoder* aDecoder)
 {
   MOZ_ASSERT(NS_IsMainThread());
-  MOZ_RELEASE_ASSERT(!mIsDoingXPCOMShutDown);
+  if (mIsDoingXPCOMShutDown) {
+    return NS_ERROR_ABORT;
+  }
   // Don't call Register() after you've Unregistered() all the decoders,
   // that's not going to work.
   MOZ_ASSERT(!mDecoders.Contains(aDecoder));
   mDecoders.PutEntry(aDecoder);
   MOZ_ASSERT(mDecoders.Contains(aDecoder));
   MOZ_ASSERT(mDecoders.Count() > 0);
+  return NS_OK;
 }
 
 void
 MediaShutdownManager::Unregister(MediaDecoder* aDecoder)
 {
   MOZ_ASSERT(NS_IsMainThread());
-  MOZ_ASSERT(mDecoders.Contains(aDecoder));
+  if (!mDecoders.Contains(aDecoder)) {
+    return;
+  }
   mDecoders.RemoveEntry(aDecoder);
   if (mIsDoingXPCOMShutDown && mDecoders.Count() == 0) {
     RemoveBlocker();
   }
 }
 
 NS_IMETHODIMP
 MediaShutdownManager::GetName(nsAString& aName)

--- a/dom/media/MediaShutdownManager.h
+++ b/dom/media/MediaShutdownManager.h
@@ -57,17 +57,17 @@ public:
   static void InitStatics();
 
   // The MediaShutdownManager is a singleton, access its instance with
   // this accessor.
   static MediaShutdownManager& Instance();
 
   // Notifies the MediaShutdownManager that it needs to track the shutdown
   // of this MediaDecoder.
-  void Register(MediaDecoder* aDecoder);
+  nsresult Register(MediaDecoder* aDecoder);
 
   // Notifies the MediaShutdownManager that a MediaDecoder that it was
   // tracking has shutdown, and it no longer needs to be shutdown in the
   // xpcom-shutdown listener.
   void Unregister(MediaDecoder* aDecoder);
 
 private: