UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:49.0) Gecko/20100101 Firefox/49.0

Example URL:
https://jsfiddle.net/8Lnuc2tf/

Steps to reproduce the problem:
1. let cStream be a CanvasCaptureStream from HTMLCanvas.captureStream(fps)
2. let aTrack be a an AudioTrack from a MediaStream (either from gUM or from WebAudioAPI)
3. call cStream.addTrack(aTrack);
4. let recorder be the result of `new MediaRecorder(cStream);`
5. attach a saveChunks function to the ondataavailable event handler

What is the expected behavior?
saveChunks should get some data in the event passed.

What went wrong?
Event's `data.size` is always 0.

Did this work before? No 

Is it a problem with Flash or HTML5? HTML5

Does this work in other browsers? Yes 

Chrome version: <Copy from: 'about:version'>  Channel: n/a
OS Version: OS X 10.9
Flash Version: Shockwave Flash 22.0 r0

It works well with a video track from gUM.
It works well when we don't add the audio track.
I also tried to use a gUM stream and replace it's video track to the canvas' one, but the same occurs. 

Firefox is not able to save a canvasStream with an audioTrack either, but it's for completely different reasons : https://bugzilla.mozilla.org/show_bug.cgi?id=1296531