Crazy fact. Firefox, for me, has ALWAYS been much faster/stronger on YouTube than any chromium based browser I've used. Better than chrome on their own site. This makes it even better. I love this browser.
YouTube has been ok for me in Firefox, but other Google apps, in particular Docs/Sheets, always become very laggy after a few minutes. When this happens, it seems to affect the rest of the browser too, so other tabs that I have open slow down as well.
Google intentionally gimps what they serve to their competition to make them look worse. It's definitely an anticompetitive practice, and they're walking a fine line about it to not get in legal trouble.