While I know Google intentionally tries to hobble its competition by making their sites run like shit on other browsers, I just can't get Youtube to play nice with Firefox. Some videos will play, but then others will just be like "playback error" fuck you. I've looked through what I can find online and nothing works, just some videos don't play, even in safe mode. Works fine in Edge and Chrome though.
Try clearing your cache. Annoying to log back into everything at once but it can fix weird things like that.