Thats Googles fault. Firefox has an user agent switcher -addon. Flip it there to appear as Chrome, and suddenly Youtube bufferring problems drastically lessen.
Also if you are in EU, consider making a complaint about this assholish and anti-competetive behaviour to your country’s competition/trade authority. Also EU’s, if you feel like being an extra responsible EU citizen. These assholes at Google need to be fined to extinction.