I have both mac and pc (6 years old) and used firefox as my main browser for a long time...Never had a problem with it. 
I can't think of what your problem could be...but if I were you, I would just uninstall firefox, flash player and toolbars (if any). Also some programs might have contributed to the problems such as Google Desktop search, live scanner etc so uninstall them as well.
Then restart and do a clean up using Ccleaner.
What virus scanner are you using?
EDIT: Oh, you might have Spybot Search & Destroy by any chance?
I can't think of what your problem could be...but if I were you, I would just uninstall firefox, flash player and toolbars (if any). Also some programs might have contributed to the problems such as Google Desktop search, live scanner etc so uninstall them as well.
Then restart and do a clean up using Ccleaner.
What virus scanner are you using?
EDIT: Oh, you might have Spybot Search & Destroy by any chance?
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