Kills that pesky AV1 but it's a shame VP9 was caught in the crossfire as YouTube preforms much worse trying to broadcast VP9 as H.264. Just use Enhanced h264ify.

I used this extension on Opera, Brave and it was OK, but in firefox it makes youtube be laggy

This does not work for anything over 1080P. Pretty much useless these days

Expected it to force 4k video to use h.264, but it just completely blocks it instead. Should've been made clear in description. The add-on only allows up to 1080p after installing.

There's a serious problem with this specific addon. If you open any youtube channel (just any channel home page, like techquickie for example), the page will just be blank.

Video and everything else loads fine with only 1080p options and stats for nerds shows avc1 is loaded, which is good.