That's rather weird because reinstalling Firefox (WITHOUT uninstalling it first) normally fixes any problems. Try again, ensuring that you've downloaded the latest STABLE version. (Do NOT use the 'beta' version of Firefox 4).
Firefox always works .. Did you remove old one?
Are you loading an unobtainable page? .. Firefox definitely does not like them!
Is it installed to the system drive? Moved any of its .ini or system files?