Firefox 3 will be out in a matter of hours now. If you're not using it already, try switching to it when the stable version is available.
You may love Firefox, but if its going haywire and a delete and install of Firefox won't fix your problem (nor the new version), try using Safari to see if you get the same problems.
Also, I've heard that Firefox tends to eat CPU like none other if the windows are open for a long time. Do you quit out of Firefox at least once a day?