I asked about the graphics card because you gave no information about it. Since you've been using it for some time, the card should be eliminated from consideration (unless you want to consider the possiblilty of impending failure of the graphics card).
I've not heard of the problem you describe with compriz-fusion, nor with Fedora 9. That leaves only Firefox 3 Beta 5 as the most likely suspect.
Have you tried the firefox support page to see if the problem has been reported by someone else?