Since late Oct 2011, my Firefox had been hanging while watching videos on Youtube. Any mouse action while a video is playing causes Firefox to hang (spinning wheel). It turns out that I had caught the beta release of the new Gray Youtube interface - where the backgrounds of all youtube pages are gray. I got myself back to the old interface by deleting all Youtube cookies. I no longer have hangs. Page transitions are also much quicker, as was before. Is there anyone else who has experienced the "new" interface, and problems? If you do have the new youtube interface, but no problems, then I have more digging to do. Update Dec 2, 2011: I have the problem again now that Youtube has released their "new look" interface to everyone. The problem is generally slow Firefox responses to mouse actions while watching Youtube videos. Firefox will hang completely 2-4 minutes into any Youtube video. (The video does continue to play a-ok.) FF becomes responsive again when the video ends playing. Problem doesn't occur when I watch a youtube video right after starting FF after having "quit" it. Problem begins occur after FF has been running for a while (visiting other websites?) Is this a symptom of FF collecting some kind of crud in memory or disk? My old Safari (3.2.1) doesn't have any problems. As far as I can see, both Firefox and Safari on my system use the same Adobe Flash Player. So, this isn't a problem with Youtube, Flash Player, or my system, but a problem with Firefox (3.6.24) !? Anybody (FF developers) have any ideas? I want to keep using FF!