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.