The ONLY Bitch I have w/ Firefox is the lastest version (9.0.1) has problems with or will NOT bring up .pdf files, They just FREEZE!
Do you use Foxit for pdf files? If not try it out. It may fix your problem.
It's probably one of your “add ons” or your copy has some how been corrupted. Save all your bookmarks on another browser like Google Chrome. Then uninstall and completely remove Firefox all traces of as if you never was on your computer. Reboot your computer and reinstall the Firefox and test it. Finally re import your bookmarks from Google Chrome.