Prefer AVC or block VP9 streams.

[mzso]

Hello!
Now that google made VP9 streams default it can cause problems. Particularly on older machines like mine. VP9 is heavier on the CPU and it can't be HW accelerated.
So a way to play AVC streams of the same resolution instead of VP9 streams is desirable in my opinion.

It will be even more problematic on firefox. (When they unbreak VP9 playback)
Because FF is both CPU heavy (especially if you have addons/scripts) and not multi-process. So playback might be jerky and will be affected by CPU spikes caused by JS and whatever from other pages and the browser's backend.

[mzso]

Someone made this for chrome:
https://chrome.google.com/webstore/detail/h264ify/aleakchihdccplidncghkekgioiakgal
https://github.com/erkserkserks/h264ify
As much as I tried it worked okay.

As for firefox you can disable media.mediasource.webm.enabled and enable media.mediasource.mp4.enabled It's like this by default for now but it won't always be.