--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -1392,16 +1392,45 @@ void HTMLMediaElement::NotifyMediaStream
     // We are a video element and HasVideo() changed so update the screen
     // wakelock
     NotifyOwnerDocumentActivityChanged();
   }
 
   mWatchManager.ManualNotify(&HTMLMediaElement::UpdateReadyStateInternal);
 }
 
+void
+HTMLMediaElement::NotifyOutputTrackStopped(DOMMediaStream* aOwningStream,
+                                           TrackID aDestinationTrackID)
+{
+  for (OutputMediaStream& ms : mOutputStreams) {
+    if (!ms.mCapturingMediaStream) {
+      continue;
+    }
+
+    if (ms.mStream != aOwningStream) {
+      continue;
+    }
+
+    for (int32_t i = ms.mTrackPorts.Length() - 1; i >= 0; --i) {
+      MediaInputPort* port = ms.mTrackPorts[i].second();
+      if (port->GetDestinationTrackId() != aDestinationTrackID) {
+        continue;
+      }
+
+      port->Destroy();
+      ms.mTrackPorts.RemoveElementAt(i);
+      return;
+    }
+  }
+
+  // An output track ended but its port is already gone.
+  // It was probably cleared by the removal of the source MediaTrack.
+}
+
 void HTMLMediaElement::LoadFromSourceChildren()
 {
   NS_ASSERTION(mDelayingLoadEvent,
                "Should delay load event (if in document) during load");
   NS_ASSERTION(mIsLoadingFromSourceChildren,
                "Must remember we're loading from source children");
 
   nsIDocument* parentDoc = OwnerDoc()->GetParentDocument();
@@ -2198,26 +2227,35 @@ class HTMLMediaElement::StreamCaptureTra
   public MediaStreamTrackSource,
   public MediaStreamTrackSource::Sink
 {
 public:
   NS_DECL_ISUPPORTS_INHERITED
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(StreamCaptureTrackSource,
                                            MediaStreamTrackSource)
 
-  explicit StreamCaptureTrackSource(MediaStreamTrackSource* aCapturedTrackSource)
+  StreamCaptureTrackSource(HTMLMediaElement* aElement,
+                           MediaStreamTrackSource* aCapturedTrackSource,
+                           DOMMediaStream* aOwningStream,
+                           TrackID aDestinationTrackID)
     : MediaStreamTrackSource(aCapturedTrackSource->GetPrincipal(),
                              nsString())
+    , mElement(aElement)
     , mCapturedTrackSource(aCapturedTrackSource)
+    , mOwningStream(aOwningStream)
+    , mDestinationTrackID(aDestinationTrackID)
   {
   }
 
   void Destroy() override
   {
-    MOZ_ASSERT(mCapturedTrackSource);
+    if (mCapturedTrackSource) {
+      mCapturedTrackSource->UnregisterSink(this);
+      mCapturedTrackSource = nullptr;
+    }
   }
 
   MediaSourceEnum GetMediaSource() const override
   {
     return MediaSourceEnum::Other;
   }
 
   CORSMode GetCORSMode() const override
@@ -2233,42 +2271,53 @@ public:
     p->Reject(new dom::MediaStreamError(aWindow,
                                         NS_LITERAL_STRING("OverconstrainedError"),
                                         NS_LITERAL_STRING("")));
     return p.forget();
   }
 
   void Stop() override
   {
-    // XXX fix in later patch
-    NS_ERROR("We're reporting remote=true to not be stoppable. "
-             "Stop() should not be called.");
+    if (mElement && mElement->mSrcStream) {
+      // Only notify if we're still playing the source stream. GC might have
+      // cleared it before the track sources.
+      mElement->NotifyOutputTrackStopped(mOwningStream, mDestinationTrackID);
+    }
+    mElement = nullptr;
+    mOwningStream = nullptr;
+
+    Destroy();
   }
 
   void PrincipalChanged() override
   {
     mPrincipal = mCapturedTrackSource->GetPrincipal();
     MediaStreamTrackSource::PrincipalChanged();
   }
 
 private:
   virtual ~StreamCaptureTrackSource() {}
 
+  RefPtr<HTMLMediaElement> mElement;
   RefPtr<MediaStreamTrackSource> mCapturedTrackSource;
+  RefPtr<DOMMediaStream> mOwningStream;
+  TrackID mDestinationTrackID;
 };
 
 NS_IMPL_ADDREF_INHERITED(HTMLMediaElement::StreamCaptureTrackSource,
                          MediaStreamTrackSource)
 NS_IMPL_RELEASE_INHERITED(HTMLMediaElement::StreamCaptureTrackSource,
                           MediaStreamTrackSource)
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION_INHERITED(HTMLMediaElement::StreamCaptureTrackSource)
 NS_INTERFACE_MAP_END_INHERITING(MediaStreamTrackSource)
 NS_IMPL_CYCLE_COLLECTION_INHERITED(HTMLMediaElement::StreamCaptureTrackSource,
                                    MediaStreamTrackSource,
-                                   mCapturedTrackSource)
+                                   mElement,
+                                   mCapturedTrackSource,
+                                   mOwningStream)
 
 class HTMLMediaElement::DecoderCaptureTrackSource :
   public MediaStreamTrackSource,
   public DecoderPrincipalChangeObserver
 {
 public:
   NS_DECL_ISUPPORTS_INHERITED
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(DecoderCaptureTrackSource,
@@ -2454,17 +2503,20 @@ HTMLMediaElement::AddCaptureMediaTrackTo
   for (auto pair : aOutputStream.mTrackPorts) {
     MOZ_ASSERT(pair.first() != aTrack->GetId(),
                "Captured track already captured to output stream");
   }
 #endif
 
   TrackID destinationTrackID = aOutputStream.mNextAvailableTrackID++;
   RefPtr<MediaStreamTrackSource> source =
-    new StreamCaptureTrackSource(&inputTrack->GetSource());
+    new StreamCaptureTrackSource(this,
+                                 &inputTrack->GetSource(),
+                                 aOutputStream.mStream,
+                                 destinationTrackID);
 
   MediaSegment::Type type = inputTrack->AsAudioStreamTrack()
                           ? MediaSegment::AUDIO
                           : MediaSegment::VIDEO;
 
   RefPtr<MediaStreamTrack> track =
     aOutputStream.mStream->CreateDOMTrack(destinationTrackID, type, source);
 

--- a/dom/html/HTMLMediaElement.h
+++ b/dom/html/HTMLMediaElement.h
@@ -350,16 +350,23 @@ public:
    */
   void NotifyMediaTrackDisabled(MediaTrack* aTrack);
 
   /**
    * Called when tracks become available to the source media stream.
    */
   void NotifyMediaStreamTracksAvailable(DOMMediaStream* aStream);
 
+  /**
+   * Called when a captured MediaStreamTrack is stopped so we can clean up its
+   * MediaInputPort.
+   */
+  void NotifyOutputTrackStopped(DOMMediaStream* aOwningStream,
+                                TrackID aDestinationTrackID);
+
   virtual bool IsNodeOfType(uint32_t aFlags) const override;
 
   /**
    * Returns the current load ID. Asynchronous events store the ID that was
    * current when they were enqueued, and if it has changed when they come to
    * fire, they consider themselves cancelled, and don't fire.
    */
   uint32_t GetCurrentLoadID() { return mCurrentLoadID; }