YouTube has quite a few bugs in Firefox. I actually prefer air, what's wrong with it?
Here's my bookmarklet (doesn't work on embedded videos in iframes, I assume because of same-origin-policy)
javascript:void%20function(){document.querySelector(%22video%22).playbackRate=parseFloat(prompt(%22Set%20the%20playback%20rate%22))}();
When there's the bandwidth to serve it, air often fails to load fast enough for 1x.