This is actually pretty great. Youtube has some issue with Firefox 43 and serves it lower quality video to deal with it (probably as a hack to enable video for some users for whom it didn't work at all under 43, but there's no explanation from Youtube engineers in the thread). The Firefox team complains to Youtube, but Youtube responds with that they won't budge until January at least.

Firefox sees it as a threat to their reputation (obviously for them having a few users for whom it doesn't work is better than having loads of users with a degraded experience) and they start spoofing their UA to another version. The Youtube engineer is indifferent (or can't really do anything about it).

Then when it's done a Google employee from a different team comes in the thread to complain about the situation because the fact that the UA is spoofed for only youtube.com and not other (Google) domains breaks his product.