--- a/dom/events/test/test_all_synthetic_events.html
+++ b/dom/events/test/test_all_synthetic_events.html
@@ -298,18 +298,19 @@ const kEventConstructors = {
   MediaKeyMessageEvent:                          { create: function (aName, aProps) {
                                                          return new MediaKeyMessageEvent(aName, aProps);
                                                        },
                                              },
   MediaStreamEvent:                          { create: function (aName, aProps) {
                                                          return new MediaStreamEvent(aName, aProps);
                                                        },
                                              },
-  MediaStreamTrackEvent:                     {
-                                               // Difficult to test required arguments.
+  MediaStreamTrackEvent:                     { create: function (aName, aProps) {
+                                                         return new MediaStreamTrackEvent(aName, aProps);
+                                                       },
                                              },
   MessageEvent:                              { create: function (aName, aProps) {
                                                          var e = new MessageEvent("messageevent", { bubbles: aProps.bubbles,
                                                              cancelable: aProps.cancelable, data: aProps.data, origin: aProps.origin,
                                                              lastEventId: aProps.lastEventId, source: aProps.source });
                                                          return e;
                                                        },
                                              },

--- a/dom/media/DOMMediaStream.cpp
+++ b/dom/media/DOMMediaStream.cpp
@@ -5,17 +5,16 @@
 
 #include "DOMMediaStream.h"
 #include "nsContentUtils.h"
 #include "nsServiceManagerUtils.h"
 #include "nsIScriptError.h"
 #include "nsIUUIDGenerator.h"
 #include "nsPIDOMWindow.h"
 #include "mozilla/dom/MediaStreamBinding.h"
-#include "mozilla/dom/MediaStreamTrackEvent.h"
 #include "mozilla/dom/LocalMediaStreamBinding.h"
 #include "mozilla/dom/AudioNode.h"
 #include "AudioChannelAgent.h"
 #include "mozilla/dom/AudioTrack.h"
 #include "mozilla/dom/AudioTrackList.h"
 #include "mozilla/dom/VideoTrack.h"
 #include "mozilla/dom/VideoTrackList.h"
 #include "mozilla/dom/HTMLCanvasElement.h"
@@ -595,21 +594,17 @@ DOMMediaStream::RemoveTrack(MediaStreamT
     LOG(LogLevel::Debug, ("DOMMediaStream %p does not contain track %p", this, &aTrack));
     return;
   }
 
   // If the track comes from a TRACK_ANY input port (i.e., mOwnedPort), we need
   // to block it in the port. Doing this for a locked track is still OK as it
   // will first block the track, then destroy the port. Both cause the track to
   // end.
-  // If the track has already ended, it's input port might be gone, so in those
-  // cases blocking the underlying track should be avoided.
-  if (!aTrack.Ended()) {
-    BlockPlaybackTrack(toRemove);
-  }
+  BlockPlaybackTrack(toRemove);
 
   DebugOnly<bool> removed = mTracks.RemoveElement(toRemove);
   MOZ_ASSERT(removed);
 
   NotifyTrackRemoved(&aTrack);
 
   LOG(LogLevel::Debug, ("DOMMediaStream %p Removed track %p", this, &aTrack));
 }
@@ -1002,19 +997,16 @@ DOMMediaStream::CreateDOMTrack(TrackID a
 
   mOwnedTracks.AppendElement(
     new TrackPort(mOwnedPort, track, TrackPort::InputPortOwnership::EXTERNAL));
 
   mTracks.AppendElement(
     new TrackPort(mPlaybackPort, track, TrackPort::InputPortOwnership::EXTERNAL));
 
   NotifyTrackAdded(track);
-
-  DispatchTrackEvent(NS_LITERAL_STRING("addtrack"), track);
-
   return track;
 }
 
 already_AddRefed<MediaStreamTrack>
 DOMMediaStream::CloneDOMTrack(MediaStreamTrack& aTrack,
                               TrackID aCloneTrackID)
 {
   MOZ_RELEASE_ASSERT(mOwnedStream);
@@ -1244,32 +1236,16 @@ DOMMediaStream::NotifyTrackRemoved(const
     mTrackListeners[i]->NotifyTrackRemoved(aTrack);
   }
 
   // Don't call RecomputePrincipal here as the track may still exist in the
   // playback stream in the MediaStreamGraph. It will instead be called when the
   // track has been confirmed removed by the graph. See BlockPlaybackTrack().
 }
 
-nsresult
-DOMMediaStream::DispatchTrackEvent(const nsAString& aName,
-                                   const RefPtr<MediaStreamTrack>& aTrack)
-{
-  MOZ_ASSERT(aName == NS_LITERAL_STRING("addtrack"),
-             "Only 'addtrack' is supported at this time");
-
-  MediaStreamTrackEventInit init;
-  init.mTrack = aTrack;
-
-  RefPtr<MediaStreamTrackEvent> event =
-    MediaStreamTrackEvent::Constructor(this, aName, init);
-
-  return DispatchTrustedEvent(event);
-}
-
 void
 DOMMediaStream::CreateAndAddPlaybackStreamListener(MediaStream* aStream)
 {
   MOZ_ASSERT(GetCameraStream(), "I'm a hack. Only DOMCameraControl may use me.");
   mPlaybackListener = new PlaybackStreamListener(this);
   aStream->AddListener(mPlaybackListener);
 }
 

--- a/dom/media/DOMMediaStream.h
+++ b/dom/media/DOMMediaStream.h
@@ -358,18 +358,16 @@ public:
   void GetTracks(nsTArray<RefPtr<MediaStreamTrack> >& aTracks) const;
   MediaStreamTrack* GetTrackById(const nsAString& aId) const;
   void AddTrack(MediaStreamTrack& aTrack);
   void RemoveTrack(MediaStreamTrack& aTrack);
 
   /** Identical to CloneInternal(TrackForwardingOption::EXPLICIT) */
   already_AddRefed<DOMMediaStream> Clone();
 
-  IMPL_EVENT_HANDLER(addtrack)
-
   // NON-WebIDL
 
   /**
    * Option to provide to CloneInternal() of which tracks should be forwarded
    * from the source stream (`this`) to the returned stream clone.
    *
    * CURRENT forwards the tracks currently in the source stream's track set.
    * ALL     forwards like EXPLICIT plus any and all future tracks originating
@@ -520,22 +518,17 @@ public:
   {
     mLogicalStreamStartTime = aTime;
   }
 
   /**
    * Called for each track in our owned stream to indicate to JS that we
    * are carrying that track.
    *
-   * Creates a MediaStreamTrack, adds it to mTracks, raises "addtrack" and
-   * returns it.
-   *
-   * Note that "addtrack" is raised synchronously and only has an effect if
-   * this MediaStream is already exposed to script. For spec compliance this is
-   * to be called from an async task.
+   * Creates a MediaStreamTrack, adds it to mTracks and returns it.
    */
   MediaStreamTrack* CreateDOMTrack(TrackID aTrackID, MediaSegment::Type aType,
                                    MediaStreamTrackSource* aSource);
 
   /**
    * Creates a MediaStreamTrack cloned from aTrack, adds it to mTracks and
    * returns it.
    * aCloneTrackID is the TrackID the new track will get in mOwnedStream.
@@ -606,20 +599,16 @@ protected:
   void NotifyTracksCreated();
 
   // Dispatches NotifyTrackAdded() to all registered track listeners.
   void NotifyTrackAdded(const RefPtr<MediaStreamTrack>& aTrack);
 
   // Dispatches NotifyTrackRemoved() to all registered track listeners.
   void NotifyTrackRemoved(const RefPtr<MediaStreamTrack>& aTrack);
 
-  // Dispatches "addtrack" or "removetrack".
-  nsresult DispatchTrackEvent(const nsAString& aName,
-                              const RefPtr<MediaStreamTrack>& aTrack);
-
   class OwnedStreamListener;
   friend class OwnedStreamListener;
 
   class PlaybackStreamListener;
   friend class PlaybackStreamListener;
 
   // XXX Bug 1124630. Remove with CameraPreviewMediaStream.
   void CreateAndAddPlaybackStreamListener(MediaStream*);

--- a/dom/media/tests/mochitest/head.js
+++ b/dom/media/tests/mochitest/head.js
@@ -436,18 +436,18 @@ function checkMediaStreamTrackCloneAgain
      "Track clone's kind should be same as the original's");
   is(clone.enabled, original.enabled,
      "Track clone's kind should be same as the original's");
 }
 
 /*** Utility methods */
 
 /** The dreadful setTimeout, use sparingly */
-function wait(time, message) {
-  return new Promise(r => setTimeout(() => r(message), time));
+function wait(time) {
+  return new Promise(r => setTimeout(r, time));
 }
 
 /** The even more dreadful setInterval, use even more sparingly */
 function waitUntil(func, time) {
   return new Promise(resolve => {
     var interval = setInterval(() => {
       if (func())  {
         clearInterval(interval);
@@ -477,18 +477,18 @@ var addFinallyToPromise = promise => {
     );
   }
   return promise;
 }
 
 /** Use event listener to call passed-in function on fire until it returns true */
 var listenUntil = (target, eventName, onFire) => {
   return new Promise(resolve => target.addEventListener(eventName,
-                                                        function callback(event) {
-    var result = onFire(event);
+                                                        function callback() {
+    var result = onFire();
     if (result) {
       target.removeEventListener(eventName, callback, false);
       resolve(result);
     }
   }, false));
 };
 
 /* Test that a function throws the right error */
@@ -604,17 +604,18 @@ function createOneShotEventWrapper(wrapp
  *        so we can avoid logging results after a test has finished.
  */
 function haveEvent(target, name, cancelPromise) {
   var listener;
   var p = Promise.race([
     (cancelPromise || new Promise()).then(e => Promise.reject(e)),
     new Promise(resolve => target.addEventListener(name, listener = resolve))
   ]);
-  return p.then(event => (target.removeEventListener(name, listener), event));
+  p.then(() => target.removeEventListener(name, listener));
+  return p;
 };
 
 /**
  * This class executes a series of functions in a continuous sequence.
  * Promise-bearing functions are executed after the previous promise completes.
  *
  * @constructor
  * @param {object} framework

--- a/dom/media/tests/mochitest/mochitest.ini
+++ b/dom/media/tests/mochitest/mochitest.ini
@@ -39,17 +39,16 @@ skip-if = toolkit == 'gonk' || buildapp 
 skip-if = toolkit == 'gonk' || buildapp == 'mulet' # b2g emulator seems to be too slow (Bug 1016498 and 1008080)
 [test_dataChannel_noOffer.html]
 [test_enumerateDevices.html]
 skip-if = buildapp == 'mulet'
 [test_getUserMedia_audioCapture.html]
 skip-if = toolkit == 'gonk' || buildapp == 'mulet' || android_version == '18' # b2g emulator seems to be too slow (Bug 1016498 and 1008080), android(Bug 1189784, timeouts on 4.3 emulator)
 [test_getUserMedia_addTrackRemoveTrack.html]
 skip-if = (toolkit == 'gonk' || buildapp == 'mulet' && debug) # debug-only failure
-[test_getUserMedia_addtrack_removetrack_events.html]
 [test_getUserMedia_basicAudio.html]
 skip-if = (toolkit == 'gonk' || buildapp == 'mulet' && debug) # debug-only failure
 [test_getUserMedia_basicVideo.html]
 skip-if = (toolkit == 'gonk' || buildapp == 'mulet' && debug) # debug-only failure
 [test_getUserMedia_basicVideo_playAfterLoadedmetadata.html]
 skip-if = (toolkit == 'gonk' || buildapp == 'mulet' && debug) # debug-only failure
 [test_getUserMedia_basicScreenshare.html]
 skip-if = buildapp == 'b2g' || toolkit == 'android' # no screenshare on b2g/android # Bug 1141029 Mulet parity with B2G Desktop for TC
@@ -79,17 +78,16 @@ skip-if = (toolkit == 'gonk' || buildapp
 [test_getUserMedia_stopVideoAudioStream.html]
 [test_getUserMedia_stopVideoAudioStreamWithFollowupVideoAudio.html]
 [test_getUserMedia_stopVideoStream.html]
 [test_getUserMedia_stopVideoStreamWithFollowupVideo.html]
 [test_getUserMedia_trackEnded.html]
 [test_getUserMedia_peerIdentity.html]
 skip-if = toolkit == 'gonk' || buildapp == 'mulet' # b2g(Bug 1021776, too --ing slow on b2g)
 [test_peerConnection_addIceCandidate.html]
-[test_peerConnection_addtrack_removetrack_events.html]
 [test_peerConnection_basicAudio.html]
 skip-if = toolkit == 'gonk' # B2G emulator is too slow to handle a two-way audio call reliably
 [test_peerConnection_basicAudioNATSrflx.html]
 skip-if = toolkit == 'gonk' || toolkit == 'android' # B2G emulator is too slow to handle a two-way audio call reliably, websockets don't work on android (bug 1266217)
 [test_peerConnection_basicAudioNATRelay.html]
 skip-if = toolkit == 'gonk' || toolkit == 'android' # B2G emulator is too slow to handle a two-way audio call reliably, websockets don't work on android (bug 1266217)
 [test_peerConnection_basicAudioNATRelayTCP.html]
 skip-if = toolkit == 'gonk' || toolkit == 'android' # B2G emulator is too slow to handle a two-way audio call reliably, websockets don't work on android (bug 1266217)

deleted file mode 100644
--- a/dom/media/tests/mochitest/test_getUserMedia_addtrack_removetrack_events.html
+++ /dev/null
@@ -1,109 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<head>
-  <script type="application/javascript" src="mediaStreamPlayback.js"></script>
-</head>
-<body>
-<pre id="test">
-<script type="application/javascript">
-"use strict";
-
-createHTML({
-  title: "MediaStream's 'addtrack' and 'removetrack' events shouldn't fire on manual operations",
-  bug: "1208328"
-});
-
-var spinEventLoop = () => new Promise(r => setTimeout(r, 0));
-
-var stream;
-var clone;
-var newStream;
-
-var addTrack = track => {
-  info("Adding track " + track.id);
-  stream.addTrack(track);
-};
-var removeTrack = track => {
-  info("Removing track " + track.id);
-  stream.removeTrack(track);
-};
-var stopTrack = track => {
-  if (track.readyState == "live") {
-    info("Stopping track " + track.id);
-  }
-  track.stop();
-};
-
-runTest(() => getUserMedia({audio: true, video: true})
-  .then(s => {
-    stream = s;
-    clone = s.clone();
-    stream.addEventListener("addtrack", function onAddtrack(event) {
-      ok(false, "addtrack fired unexpectedly for track " + event.track.id);
-    });
-    stream.addEventListener("removetrack", function onRemovetrack(event) {
-      ok(false, "removetrack fired unexpectedly for track " + event.track.id);
-    });
-
-    return getUserMedia({audio: true, video: true});
-  })
-  .then(s => {
-    newStream = s;
-
-    info("Stopping an original track");
-    stopTrack(stream.getTracks()[0]);
-
-    return spinEventLoop();
-  })
-  .then(() => {
-    info("Removing original tracks");
-    stream.getTracks().forEach(t => stream.removeTrack(t));
-
-    return spinEventLoop();
-  })
-  .then(() => {
-    info("Adding other gUM tracks");
-    newStream.getTracks().forEach(t => addTrack(t))
-
-    return spinEventLoop();
-  })
-  .then(() => {
-    info("Adding cloned tracks");
-    let clone = stream.clone();
-    clone.getTracks().forEach(t => addTrack(t));
-
-    return spinEventLoop();
-  })
-  .then(() => {
-    info("Removing a clone");
-    removeTrack(clone.getTracks()[0]);
-
-    return spinEventLoop();
-  })
-  .then(() => {
-    info("Stopping clones");
-    clone.getTracks().forEach(t => stopTrack(t));
-
-    return spinEventLoop();
-  })
-  .then(() => {
-    info("Stopping originals");
-    stream.getTracks().forEach(t => stopTrack(t));
-
-    return spinEventLoop();
-  })
-  .then(() => {
-    info("Removing remaining tracks");
-    stream.getTracks().forEach(t => removeTrack(t));
-
-    return spinEventLoop();
-  })
-  .then(() => {
-      // Test MediaStreamTrackEvent required args here.
-      mustThrowWith("MediaStreamTrackEvent without required args",
-                    "TypeError", () => new MediaStreamTrackEvent("addtrack", {}));
-  }));
-</script>
-</pre>
-</body>
-</html>

deleted file mode 100644
--- a/dom/media/tests/mochitest/test_peerConnection_addtrack_removetrack_events.html
+++ /dev/null
@@ -1,70 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<head>
-  <script type="application/javascript" src="pc.js"></script>
-</head>
-<body>
-<pre id="test">
-<script type="application/javascript">
-"use strict";
-
-createHTML({
-  title: "MediaStream's 'addtrack' and 'removetrack' events with gUM",
-  bug: "1208328"
-});
-
-runNetworkTest(function (options) {
-  let test = new PeerConnectionTest(options);
-  let eventsPromise;
-  addRenegotiation(test.chain,
-    [
-      function PC_LOCAL_SWAP_VIDEO_TRACKS(test) {
-        return getUserMedia({video: true}).then(stream => {
-          let localStream = test.pcLocal._pc.getLocalStreams()[0];
-          let remoteStream = test.pcRemote._pc.getRemoteStreams()[0];
-
-          let newTrack = stream.getTracks()[0];
-
-          let videoSenderIndex =
-            test.pcLocal._pc.getSenders().findIndex(s => s.track.kind == "video");
-          isnot(videoSenderIndex, -1, "Should have video sender");
-
-          test.pcLocal.removeSender(videoSenderIndex);
-          test.pcLocal.attachLocalTrack(stream.getTracks()[0], localStream);
-
-          let onNextLoop = wait(0);
-          eventsPromise = haveEvent(remoteStream, "addtrack", wait(50000, "No addtrack event"))
-            .then(trackEvent => {
-              ok(trackEvent instanceof MediaStreamTrackEvent,
-                 "Expected event to be instance of MediaStreamTrackEvent");
-              is(trackEvent.type, "addtrack",
-                 "Expected addtrack event type");
-              is(trackEvent.track.id, newTrack.id, "Expected track in event");
-              is(trackEvent.track.readyState, "live",
-                 "added track should be live");
-            })
-            .then(() => haveEvent(remoteStream, "addtrack", onNextLoop)
-              .then(() => Promise.reject("Unexpected addtrack event for remote stream " + remoteStream.id),
-                    () => Promise.resolve())
-            );
-          remoteStream.addEventListener("removetrack",
-                                        function onRemovetrack(trackEvent) {
-            ok(false, "UA shouldn't raise 'removetrack' when receiving peer connection");
-          })
-        });
-      },
-    ],
-    [
-      function PC_REMOTE_CHECK_EVENTS(test) {
-        return eventsPromise;
-      },
-    ]
-  );
-
-  test.setMediaConstraints([{audio: true, video: true}], []);
-  test.run();
-});
-</script>
-</pre>
-</body>
-</html>

--- a/dom/webidl/MediaStream.webidl
+++ b/dom/webidl/MediaStream.webidl
@@ -38,12 +38,12 @@ interface MediaStream : EventTarget {
     sequence<MediaStreamTrack> getTracks ();
     MediaStreamTrack?          getTrackById (DOMString trackId);
     void                       addTrack (MediaStreamTrack track);
     void                       removeTrack (MediaStreamTrack track);
     MediaStream                clone ();
     // readonly    attribute boolean      active;
     //             attribute EventHandler onactive;
     //             attribute EventHandler oninactive;
-                attribute EventHandler onaddtrack;
+    //             attribute EventHandler onaddtrack;
     //             attribute EventHandler onremovetrack;
     readonly attribute double currentTime;
 };

--- a/dom/webidl/MediaStreamTrackEvent.webidl
+++ b/dom/webidl/MediaStreamTrackEvent.webidl
@@ -3,17 +3,20 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/.
  *
  * The origin of this IDL file is
  * http://dev.w3.org/2011/webrtc/editor/webrtc.html#mediastreamevent
  */
 
 dictionary MediaStreamTrackEventInit : EventInit {
-    required MediaStreamTrack track;
+  MediaStreamTrack? track = null;
+  RTCRtpReceiver? receiver = null;
+  MediaStream? stream = null;
 };
 
-[Exposed=Window,
- Constructor (DOMString type, MediaStreamTrackEventInit eventInitDict)]
+[Pref="media.peerconnection.enabled",
+ Constructor(DOMString type, optional MediaStreamTrackEventInit eventInitDict)]
 interface MediaStreamTrackEvent : Event {
-    [SameObject]
-    readonly        attribute MediaStreamTrack track;
+  readonly attribute RTCRtpReceiver? receiver;
+  readonly attribute MediaStreamTrack? track;
+  readonly attribute MediaStream? stream;
 };