First I would check and make sure you computer meets the requirements for Firefox (if you bought it within the last few years it should be fine). Then I would upgrade to the newest version of Firefox (1.5). If that still fails, unistall it completey and re-install. Check your other programs and see if there are any compatibility problems.
Firefox browser causes my CPU useage to go to 100% and stay there. No "add ons" to the browser installed.