Try uninstalling. And going to the registry and removing all entries of firefox. Then go into explorer and totally delete the firefox folder unless it is already removed when uninstalled. As that guy said I think its a profile issue just as there are similar pofile issues when profiles on xp are corrupt. But yeah my guess is that your default profile is corrupt and I dont think it is removed when firefox is uninstalled.
shut down firefox, go to the task manager to make sure it is off start >> run >> type in firefox.exe -profilemanager and erol, stfu, no one wants to hear about opera, personaly, FF>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>opera