Are they? I watch YouTube on Firefox all the time, seems fine on my machine.
I think maybe 5+ years ago there were some performance issues caused by YT relying on features that were only implemented in Chrome, but I don't recall having any issues wrt that for years.
I wouldn't say "completely fucked", but for a few years I noticed YouTube on Firefox has this occasional quirk where videos will quit playing and infinitely buffer at the exact same timestamp. Like there's no way around it except skipping about 30 seconds ahead with the seek bar, or doing a Ctrl-F5 (hard refresh) and starting the whole video over. Opera GX doesn't seem to have this problem at all.
But it's still not a big enough deal to make me give up Firefox completely.
I've got uBlock and Privacy Badger but turning them off or going incognito doesn't help at all. The most common issue I get with YouTube is the video keeps freezing. Apparently this is because Google deliberately fuck it so that other browsers have to play catch up constantly. I have heard this is why Microsoft gave up and adopted Chromium.
The other issue is that if I open more than one YouTube tab my laptop sounds like it is about to take off into space. I can have an unlimited number of tabs from any other website open though.
Sounds to me like some hardware issue, I've literally never experienced any of this in the last 5 years on Firefox. My guess is considering it works fine with other browsers the graphics drivers are a bit wonky, or maybe Firefox is falling back to software rendering for some reason. Are you using Linux or Windows?
Strange, usually things just work there considering the limited hardware variety. Is it an older Mac? I'm typing this on an M2 macbook and it works perfectly.