Google has teams of highly paid expert engineers who’s entire job is to maintain and develop youTube. What do you think is more likely:
Google’s engineers were unable to tell that performance in Firefox is degraded by their changes.
Google sees it as advantageous to disadvantage their competitors - including Firefox. And although they might not be able to do it deliberately, for legal reasons, they can still do it by introducing platform specific changes and strategically neglecting to make it work properly.