[0x006A]

Firefox supports VP9 but currently does not enable VP9 with MediaSourceExtensions (MSE) by default. You can enable the current implementation via a flag set media.mediasource.webm.enabled to true in about:config

Youtube only used VP9 with MSE but even then not all content is available as VP9, some is only available as H.264/MSE.

You can check https://www.youtube.com/html5 to see what your browser supports.

[piyush_soni]

Thanks. I did all of that, and I can see that all the formats are checked in my Firefox now. The only question now I have is, how do I know which of these formats is my YouTube video currently playing in? (It does show the HTML5 player now).

[mkagenius]

Right click on video --> stats for nerds --> MimeType will show codecs as vp9.

offtopic: I could see download rate at 9363 Kbps (or nearly 1.2 MBPS) for a 1080p video, which looks like pretty good compression to me.

[edwintorok]

FWIW after enabling MSE, playing a 4k video, and opening 'stats for nerds' and switching away and back to firefox (Iceweasel 31.6.0) on Debian jessie the browser locks up at 100% cpu usage.

[piyush_soni]

Wow. Thanks, I can see it's playing in vp9 now :).

[mrinterweb]

Thanks for the info. After visiting the Youtube HTML5 support page and clicking "Request the HTML 5 player", and enabling the setting in FF, the VP9 playback is working in FF.