I was using Firefox 4.0 when I 1st started having this problem. I downloaded version 5.0 to see if it would fix the problem for me, but it didn't. Not long after I start checking my e-mails and going to different websites, my computer will work really slow and show that I'm using 100% of my CPU usage. It used to be a flash plug-in that was using all of my CPU usage, but I read on here somewhere that I could disable that, at the risk of Firefox crashing instead of just the plug-in. So I disabled the flash plug-in that was always hanging up. Now Firefox is always the process that is using the most CPU usage, from anywhere between 75% and 99%. I end up having to bring up my task manager and closing the Firefox process, wait a little while, and then I open a Firefox window again. Then after about 10-15 min. it does it again. I've read in the help knowledge base for Firefox that people are having better luck using Firefox 3.6. I don't know how to go about getting rid of 5.0 and reloading 3.6, or if that's even the best solution.