I don't know about 100% CPU usage, but I've been having other problems with Firefox. Half the time I start it up, it comes up with a message saying that there is a problem with the flash player and recommends closing firefox. Regardless of whether I close it or not (since eventually it just ceases to function), when I try to re-open it, it says that there is already an instance of Firefox running, and won't allow it to open another window. I either have to close Firefox through the task manager, or restart windows. This has been happening for over a month now, and even though we recently updated both Firefox and the Flash player, it still has this problem.

Any ideas why?