diff --git a/dom/media/MediaDecoderStateMachine.cpp b/dom/media/MediaDecoderStateMachine.cpp
index d682c35ecee0..5df1f793b494 100644
--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -205,11 +205,12 @@ MediaDecoderStateMachine::MediaDecoderStateMachine(MediaDecoder* aDecoder,
   mRealTime(aRealTime),
   mDispatchedStateMachine(false),
   mDelayedScheduler(this),
-  mState(DECODER_STATE_DECODING_NONE),
+  mState(DECODER_STATE_DECODING_NONE, "MediaDecoderStateMachine::mState"),
   mPlayDuration(0),
   mStartTime(-1),
   mEndTime(-1),
   mDurationSet(false),
+  mNextFrameStatusUpdater("MediaDecoderStateMachine::mNextFrameStatusUpdater"),
   mFragmentEndTime(-1),
   mReader(aReader),
   mCurrentFrameTime(0),
@@ -228,7 +229,7 @@ MediaDecoderStateMachine::MediaDecoderStateMachine(MediaDecoder* aDecoder,
   mIsVideoPrerolling(false),
   mAudioCaptured(false),
   mPositionChangeQueued(false),
-  mAudioCompleted(false),
+  mAudioCompleted(false, "MediaDecoderStateMachine::mAudioCompleted"),
   mGotDurationFromMetaData(false),
   mDispatchedEventToDecode(false),
   mStopAudioThread(true),
@@ -257,6 +258,12 @@ MediaDecoderStateMachine::MediaDecoderStateMachine(MediaDecoder* aDecoder,
   mNextFrameStatus.Init(mTaskQueue, MediaDecoderOwner::NEXT_FRAME_UNINITIALIZED,
                         "MediaDecoderStateMachine::mNextFrameStatus (Canonical)");
 
+  // Skip the initial notification we get when we Watch the value, since we're
+  // not on the right thread yet.
+  mNextFrameStatusUpdater->Watch(mState, /* aSkipInitialNotify = */ true);
+  mNextFrameStatusUpdater->Watch(mAudioCompleted, /* aSkipInitialNotify = */ true);
+  mNextFrameStatusUpdater->AddWeakCallback(this, &MediaDecoderStateMachine::UpdateNextFrameStatus);
+
   static bool sPrefCacheInit = false;
   if (!sPrefCacheInit) {
     sPrefCacheInit = true;
@@ -1321,8 +1328,6 @@ void MediaDecoderStateMachine::SetState(State aState)
 
   mState = aState;
 
-  UpdateNextFrameStatus();
-
   // Clear state-scoped state.
   mSentPlaybackEndedEvent = false;
 }
@@ -3182,7 +3187,7 @@ void MediaDecoderStateMachine::SetStartTime(int64_t aStartTimeUsecs)
 
 void MediaDecoderStateMachine::UpdateNextFrameStatus() {
   MOZ_ASSERT(OnTaskQueue());
-  AssertCurrentThreadInMonitor();
+  ReentrantMonitorAutoEnter mon(mDecoder->GetReentrantMonitor());
 
   MediaDecoderOwner::NextFrameStatus status;
   const char* statusString;
@@ -3424,7 +3429,6 @@ void MediaDecoderStateMachine::OnAudioSinkComplete()
   }
   ResyncAudioClock();
   mAudioCompleted = true;
-  UpdateNextFrameStatus();
   // Kick the decode thread; it may be sleeping waiting for this to finish.
   mDecoder->GetReentrantMonitor().NotifyAll();
 }
diff --git a/dom/media/MediaDecoderStateMachine.h b/dom/media/MediaDecoderStateMachine.h
index 521e09b0c59d..40fab5faaa08 100644
--- a/dom/media/MediaDecoderStateMachine.h
+++ b/dom/media/MediaDecoderStateMachine.h
@@ -844,7 +844,7 @@ public:
   // NotifyAll on the monitor must be called when the state is changed so
   // that interested threads can wake up and alter behaviour if appropriate
   // Accessed on state machine, audio, main, and AV thread.
-  State mState;
+  Watchable<State> mState;
 
   // The task queue in which we run decode tasks. This is referred to as
   // the "decode thread", though in practise tasks can run on a different
@@ -887,6 +887,7 @@ public:
 
   // The status of our next frame. Mirrored on the main thread and used to
   // compute ready state.
+  WatcherHolder mNextFrameStatusUpdater;
   Canonical<NextFrameStatus>::Holder mNextFrameStatus;
 public:
   AbstractCanonical<NextFrameStatus>* CanonicalNextFrameStatus() { return &mNextFrameStatus; }
@@ -1155,7 +1156,7 @@ protected:
   // the state machine thread. Synchronised via decoder monitor.
   // When data is being sent to a MediaStream, this is true when all data has
   // been written to the MediaStream.
-  bool mAudioCompleted;
+  Watchable<bool> mAudioCompleted;
 
   // True if mDuration has a value obtained from an HTTP header, or from
   // the media index/metadata. Accessed on the state machine thread.
diff --git a/dom/media/StateWatching.h b/dom/media/StateWatching.h
index ac8052d1d85a..bfef7488a0b1 100644
--- a/dom/media/StateWatching.h
+++ b/dom/media/StateWatching.h
@@ -70,7 +70,7 @@ void EnsureStateWatchingLog();
 class AbstractWatcher
 {
 public:
-  NS_INLINE_DECL_REFCOUNTING(AbstractWatcher)
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AbstractWatcher)
   AbstractWatcher() : mDestroyed(false) {}
   bool IsDestroyed() { return mDestroyed; }
   virtual void Notify() = 0;
@@ -104,11 +104,13 @@ class WatchTarget
 public:
   explicit WatchTarget(const char* aName) : mName(aName) {}
 
-  void AddWatcher(AbstractWatcher* aWatcher)
+  void AddWatcher(AbstractWatcher* aWatcher, bool aSkipInitialNotify)
   {
     MOZ_ASSERT(!mWatchers.Contains(aWatcher));
     mWatchers.AppendElement(aWatcher);
-    aWatcher->Notify();
+    if (!aSkipInitialNotify) {
+      aWatcher->Notify();
+    }
   }
 
   void RemoveWatcher(AbstractWatcher* aWatcher)
@@ -204,7 +206,7 @@ public:
     AbstractThread::GetCurrent()->TailDispatcher().AddDirectTask(r.forget());
   }
 
-  void Watch(WatchTarget& aTarget) { aTarget.AddWatcher(this); }
+  void Watch(WatchTarget& aTarget, bool aSkipInitialNotify = false) { aTarget.AddWatcher(this, aSkipInitialNotify); }
   void Unwatch(WatchTarget& aTarget) { aTarget.RemoveWatcher(this); }
 
   template<typename ThisType>