[acdha]

Or just go into about:config and set media.webm.enabled to false.

Set media.mediasource.enabled=true to enable MSE and everything on https://www.youtube.com/html5 will be enabled except for VP8/9.

[lbenes]

This will only disable webm VP8 + VP9, forcing FF to fallback to H264.

In order to enable MSE & WebM VP9 in Firefox nightly, you must leave:

media.webm.enabled=true (default)

media.mediasource.enabled=true (default)

The only value you must change is:

media.mediasource.webm.enabled=true (user-set)

[acdha]

> This will only disable webm VP8 + VP9, forcing FF to fallback to H264.

I'm glad you edited your comment but I'm still not sure what you disagree about. SoapSeller was talking about hardware acceleration and in that context disabling WebM globally will provide better performance/battery-life.

The only thing I was contributing to the discussion was the note that on Firefox this does not require an extension.

[ubercow13]

Wouldn't that mean you cant use webm on other sites, though?

[acdha]

Correct. If your goal is the improved performance and battery-life, that's probably acceptable but it's definitely something to think about if you frequent sites which don't support H.264.