YouTube now defaults to AV1. The extension was last updated 6 years ago, so it still only blocks VP8/VP9.

EDIT: After changing this setting to false in about:config it works: media.av1.enabled

A must have for old laptops with no VP9 hardware decoding support. Just make sure that you can only watch videos up to 1080p.

Doesn't seem to be working any more?... I just turned it on, restarted Firefox, and checked stats on a YT video, still says vp09

It does block VP9 but I didn't notice a change in CPU usage...

its good but it dont support higher than 1080p

Doesn't really work. It's always avc codec, no matter what setting is set in h264ify, and when opening new youtube tabs 60fps isn't blocked.
Please fix?
UPDATE: Hmm, sometimes it works but not all of the time? Surely this needs some fixing. 4 out of 5 until then :)