YouTube reports: "When playing a H.264 video with frame packing arrangement set in Firefox on Windows, the video will be stretched. E.g., a 1920*1080 video will be playing as 3840*1080 video, if the FPA is set to side by side. This might be due to Firefox calling internal decoder in Windows. Playing the same video via Windows Media Player would result in the same behaviour." https://www.youtube.com/watch?v=5B4-HqlD3L0 See the attached screenshot comparing Chrome 3D (left) vs Firefox 3D (right).

(In reply to Chris Peterson [:cpeterson] from comment #0) > "When playing a H.264 video with frame packing arrangement set in Firefox on > Windows, the video will be stretched. E.g., a 1920*1080 video will be > playing as 3840*1080 video, if the FPA is set to side by side. This might be > due to Firefox calling internal decoder in Windows. Playing the same video > via Windows Media Player would result in the same behaviour." Firefox only supports 3D video in WebM. Disabling MP4 shows the video as being narrower but it still appears different to what we get in Chrome.

I don't see the red/blue 3D video when playing WebM on OS X. I set media.apple.mp4.enabled = false and verified that YouTube served video/webm; codecs="vp8.0, vorbis".

Its also not working with WebM/MSE enabled (media.mediasource.webm.enabled). Which would be used once bug 1198715 is closed.

(In reply to Jan Gerber from comment #3) > Its also not working with WebM/MSE enabled (media.mediasource.webm.enabled). > Which would be used once bug 1198715 is closed. In that case, we should probably fix this 3D bug before we enable WebM/MSE in bug 1198715.

(In reply to Chris Peterson [:cpeterson] from comment #4) > In that case, we should probably fix this 3D bug before we enable WebM/MSE > in bug 1198715. This is more an issue with the new MSE code. WebMDemuxer passes the stereo mode along, not sure where it is lost. 3D is also broken with H.264/MSE

(In reply to Jan Gerber from comment #5) > 3D is also broken with H.264/MSE 3D is only supported on WebM. Nothing is broken.

Just tested with Chrome 45.0.2454.85 and it displays the video in the same way Firefox with VP9/MSE does here: 2 videos side-by-side squashed to 4:3. Looking at the 480p video with mkvinfo, Stereo mode is not set. Could it be that 3D is broken in general on YouTube?

WFM with WebM on Nightly 50: https://www.youtube.com/watch?v=FSGfN9rr78Q