You may be thinking that you’ve done everything to stop this nasty habit of Firefox. But have you tried this? I’ve done it in Firefox 2.0.0.3, but it will work with all other versions of Firefox too. OK, here’s the trick.
1. Type about:config in the address bar and press Enter to open the Firefox expert configuration page.
2. Type browser.cache.disk.capacity in the Filter text box.
3. Change 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.