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.
Weird, I'd check the debug console for anything obvious and probably reinstall (removing the existing profile).
Personally I'm using Waterfox ever since some change or other regarding addons, but it's been probably a decade since I had any browser, Chromium/Waterfox/Firefox/whatever even crash or have a bad memory leak (unless I've got 700 tabs open because I don't close it for 3 weeks).