You may be thinking that you have done all the things to stop this nasty habit of Firefox. But have you tried this? I’ve done this is Firefox 2.0.0.3 but this will work with all the other versions of Firefox. Ok! here’s the trick.
1. Type about:config in the address bar and press Enter to get the Firefox expert configuration page.
2. Type browser.cache.disk.capacity in the Filter text box.
3. Modify the value to 16384
4. Restart Firefox and there you go!
After doing this, CPU usage for Firefox should stay under 10%
-SeeJay
SeeJay Online! 
Thx. I’ll try this trick. Coz I’ve experienced the problems with CPU load recently.