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.
Doesnt seem to work for many people (Cloudflare has stopped supporting it?), judging by reading reviews on Mozilla extension store.