The link posted above by Saab Dastard gives the 'Profiles' folder in which your profile is stored. If you create a copy of the 'nnnnnnn.default' folder (in which nnnnnnnn is a random string which is created by FF) you can get all your settings back.
After creating the backup, reinstall Firefox, find the folder 'Profiles' again, copy the folder back into this folder (it may actually still be there if you've just copied it) and edit the profiles.ini file using notepad. The last line in this file reads:
Path=Profiles/nnnnnnnn.default
Make sure that nnnnnnnn is changed to point to the folder you've just copied. Save, open Firefox and you should be back in business.
The tip in the post above is also useful and often used by me to get FF behaving again.