Firefox's implementation of Media Source Extensions took a step forward recently. With last week's landing of the patch sets from bug 881512 and bug 1000608, along with a handful of minor follow-ups, videos are now playable in YouTube's HTML5 DASH player. The functionality in these patches extends Firefox's general Media Source Extensions implementation for all users, but the current focus for this iteration of development has been playback with YouTube's player.

It's now at a point that wider testing and technical feedback would be useful, so if you're feeling brave and need a good excuse to watch cats on treadmills and file bugs, please try it out. For now, MSE is hidden behind a preference, so first you'll need to set the preference to true:

about:config screenshot

Next, make sure YouTube detects MSE support by visiting YouTube's HTML5 feature test page, which should look like this:

YouTube HTML5

Now your Firefox is ready to test MSE. To verify that you're using MSE on a YouTube video, right-click on the video area and select "Stats for nerds" from the pop-up menu, which will indicate "DASH: yes" if MSE being used with the current video. Note any unusual behaviour that you haven't seen with the HTML5 player, bearing in mind the caveats listed below.

YouTube DASH

Not all videos have DASH versions, so here's a video I've been testing:

Before filing new bugs, please be aware of the following known bugs and limitations:

If you find a bug, please file it in Bugzilla against Core::Video/Audio, add "mediasource" (aka bug 778617) to the "Blocks" field, and include me (:kinetik) in the CC list.

Once the first two of the issues listed above have been fixed, along with any other blocking bugs that may be found during testing, MSE will be enabled by default (for WebM only) in Nightly builds for further testing. That change is tracked by bug 1000686.