Last Friday everything worked fine. Today, if I try to watch a video while signed in to Google account I can only do it with quality set to auto that then defaults to 144p or 240p. Setting quality to 360p or higher results in constant buffering like we're back in 2008 and 720p does not even start playing. Tried Firefox on my work computer at work, Librewolf on my personal machine at home, both machines are at different ISP-s, work DL speed is 50 Mbit/s, home is 150. Tried disabling uBlock Origin, tried clearing browser cache, no go. Only way I can get videos to play in FF is when I log out or open them in private window. Cromium-based Vivaldi works without a hitch, too, with uBlock, Tampermonkey and all that jazz active.
What in the Seven Hells is going on here, Google? Has my account been flagged and throttled for refusing to disable adblock? Then why does it work fine on Chromium with adblock? Is FF under attack by Google and they are throttling YT? Then why everything works fine when signed out from Google? Why it started happening suddenly over the weekend? So many questions...
It seems to be working for me, so maybe it's some Google A&B testing?
Try switching your user agent to chrome to see if it fixes it, that might be proof of some trickery