I'm running Flash player 11,8,800,94 with Firefox 23.0.1 under Window 7 Pro 64-bit (with all the latest bug fixes) on a Dell Studio 1558 (core 4 CPU) and have this problem: After viewing a Flash movie under Firefox, my system slows to a crawl, while CPU usage (per Task Manager) shows nearly 100% -- even after I quit Firefox -- and even when I am doing only mundane things like opening a text file in Notepad. Only re-booting helps then.
I followed these instructions [http://forums.adobe.com/thread/1018071] to disable Flash protected mode. Not only did this not fix my problem, it induced a new one: After viewing 5 to 10 minutes of flash movies, my system would suddenly power off with no warning I removed the line "ProtectedMode=0" which I had added to C:\Windows\SysWOW64\Macromed\Flash\mms.cfg and the sudden power-off problem no longer recurred.
So I completely uninstalled Flash Player and the problem did not recur--of course, since it only happens after playing a Flash movie. But then again I couldn't play Flash movies. So I reinstalled Flash player--the latest version, straight from Adobe--and as soon as I played my first movie, the CPU usage shot up and stayed up around 80% even though I was doing nothing! I killed the Flash Player Plugin from Task Manager and the CPU usage did not drop. Task Manager says that it's all going to ordinary things that shouldn't take any time, like an idle Firefox session and even 14% to Task Manager itself. Something lurking in my computer is making everything inefficient. It will be OK after I reboot, until I play my next Flash video.
By the way, this problem only happens after playing Flash videos through the Adobe Flash player plugin to Firefox. I played some FLV files using the stand-alone FLV Player from Applian (based on the VLC player), and had no troubles at all.
So, the obvious questions are, why is this happening and how can I prevent it? I'm getting tired of re-booting just to get my computer back after each web site which pops up a movie.