Try starting in Firefox's Safe Mode (shortcut in the Start Menu). Even though there are no extensions or themes installed it might as well be tried.
Many (or even most) of Firefox's problems stem from the Profiles folder. Perhaps create a new Profile (in addition to the one currently in use) for test purposes. It can always be deleted if it doesn't do any good ...or kept and built up to requirements if it does. The Profile is not replaced or affected by reinstalling FF.
To start the Profile Manager and create an additional Profile:
Go to: Start > Run. Type:
firefox.exe -p and press OK. Create and give a name to the new Profile. Untick the box that says, 'Don't ask at startup' to make the Profile Manager start each time the FF shortcut is ticked. Choose which Profile FF should start with.
Edit: The above might be relevant but just looking into the FF forum and saw this thread:
http://forums.mozillazine.org/viewtopic.php?t=325321