Browsers identify themselves with a user-agent string, and the user agent strings are a giant disaster of technical debt resulting from a long history of browsers working around buggy websites and websites working around buggy browsers. Browser video is also a giant disaster of technical debt, and YouTube uses the user-agent string to determine which ways of serving video will work and which ones won't. Presence of bugs here is no surprise, and any such bugs will naturally impact some browsers but not others.
So, a bug in YouTube's compatibility-autodetection caused it to fall back to a slower, bitrate-limited fallback method that it didn't need to, for some versions of Firefox. They took two weeks to deploy the fix. Considering those two weeks included Christmas and the bug didn't affect security or render the site unavailable, this seems like a pretty good response time.
So, a bug in YouTube's compatibility-autodetection caused it to fall back to a slower, bitrate-limited fallback method that it didn't need to, for some versions of Firefox. They took two weeks to deploy the fix. Considering those two weeks included Christmas and the bug didn't affect security or render the site unavailable, this seems like a pretty good response time.