--- a/dom/media/mediasource/MediaSourceDecoder.cpp
+++ b/dom/media/mediasource/MediaSourceDecoder.cpp
@@ -187,17 +187,22 @@ MediaSourceDecoder::DetachMediaSource()
   mMediaSource = nullptr;
 }
 
 void
 MediaSourceDecoder::Ended(bool aEnded)
 {
   MOZ_ASSERT(NS_IsMainThread());
   static_cast<MediaSourceResource*>(GetResource())->SetEnded(aEnded);
-  mEnded = true;
+  if (aEnded) {
+    // We want the MediaSourceReader to refresh its buffered range as it may
+    // have been modified (end lined up).
+    NotifyDataArrived();
+  }
+  mEnded = aEnded;
 }
 
 void
 MediaSourceDecoder::AddSizeOfResources(ResourceSizes* aSizes)
 {
   MOZ_ASSERT(NS_IsMainThread());
   if (GetDemuxer()) {
     GetDemuxer()->AddSizeOfResources(aSizes);
@@ -305,16 +310,18 @@ MediaSourceDecoder::CanPlayThrough()
                         timeAhead,
                         MediaSourceDemuxer::EOS_FUZZ);
   return GetBuffered().Contains(ClampIntervalToEnd(interval));
 }
 
 TimeInterval
 MediaSourceDecoder::ClampIntervalToEnd(const TimeInterval& aInterval)
 {
+  MOZ_ASSERT(NS_IsMainThread());
+
   if (!mEnded) {
     return aInterval;
   }
   TimeInterval interval(TimeUnit(), TimeUnit::FromSeconds(GetDuration()));
   return aInterval.Intersection(interval);
 }
 
 #undef MSE_DEBUG

--- a/dom/media/mediasource/MediaSourceDecoder.h
+++ b/dom/media/mediasource/MediaSourceDecoder.h
@@ -86,14 +86,14 @@ private:
 
   // The owning MediaSource holds a strong reference to this decoder, and
   // calls Attach/DetachMediaSource on this decoder to set and clear
   // mMediaSource.
   dom::MediaSource* mMediaSource;
   RefPtr<MediaSourceDemuxer> mDemuxer;
   RefPtr<MediaFormatReader> mReader;
 
-  Atomic<bool> mEnded;
+  bool mEnded;
 };
 
 } // namespace mozilla
 
 #endif /* MOZILLA_MEDIASOURCEDECODER_H_ */

--- a/dom/media/mediasource/SourceBuffer.cpp
+++ b/dom/media/mediasource/SourceBuffer.cpp
@@ -284,19 +284,16 @@ SourceBuffer::Detach()
 
 void
 SourceBuffer::Ended()
 {
   MOZ_ASSERT(NS_IsMainThread());
   MOZ_ASSERT(IsAttached());
   MSE_DEBUG("Ended");
   mTrackBuffersManager->Ended();
-  // We want the MediaSourceReader to refresh its buffered range as it may
-  // have been modified (end lined up).
-  mMediaSource->GetDecoder()->NotifyDataArrived();
 }
 
 SourceBuffer::SourceBuffer(MediaSource* aMediaSource, const nsACString& aType)
   : DOMEventTargetHelper(aMediaSource->GetParentObject())
   , mMediaSource(aMediaSource)
   , mCurrentAttributes(aType.LowerCaseEqualsLiteral("audio/mpeg") ||
                        aType.LowerCaseEqualsLiteral("audio/aac"))
   , mUpdating(false)