The various websites will just say Firefox is "not supported". I just wrote this in another comment, but Twitch doesn't let you log in on FF because it has some kind of advanced tracking protection. I guess YouTube and the rest will just join the fuckery and block you from using their content if you're on FF. I mean, I really hope they won't do that, but knowing what degree of assholery these companies can pull off, I think it's the next step.