the very problematic plugin-cointainer is the cause of all this! i assume you may be open web page that contain flash right? high system resource slowing your browsing capabilities (click,tab,openpage, slowresponse,etc) it is very common problem in windows. you can read it in here under Known Issues->http://www.mozilla.com/en-US/firefox/3.6.15/releasenotes/ +Some slower machines may see degraded mouse responsiveness when viewing certain Flash videos This PROBABLY can happen in your macosx too. Im not sure for that. I also try firefox in Ubuntu, and suprisely it smoothly open flash video and playing FB Poker at the same time wo performance degradation . i cant give any solution for this unless they came out with better and enhanced performance of firefox for all systems.