It will work but just so you know, this addon removes the settings for above 1080p video quality (like 4K).
It also seems like it has been abandoned by it's current author.

Fixed youtube high CPU usage. I think Mozilla should add this extension to recommended ones.
PS. For your knowledge the max resolution you will get using this extension is 1080p.

fixed the long-time issue with youtube stutters/lags over time

this is absolutely amazing.

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

при подключении этого расширения всё стало только больше тормозить, в других сервисах всё работает нормально, так что беда не в интернете

My Firefox was having issues with YouTube videos lately. It's double buffering at the start of video playback and seems to be stuttering on some parts, like skipping or going back a second. It only started happening with FF 96 updates.

I got so annoyed that it wasn't happening with my secondary GX browser. I started searching and found a thread leading to this addon and bam. It fixed the very problems i was having. No more buffering, no more stuttering/lag/skip, etc. Thank you so very much.

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

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.

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