You should completely reinstall it, ergo:

  1. Uninstall Firefox
  2. Delete the Firefox directory/folder (default: C://Program Files/Firefox or something like it)
  3. Delete the Firefox directory/folder in My Documents
  4. Run a search for all files containing the word "Firefox" in them and see if you need those files, if not delete or rename them
  5. Run a virus scan with Dr. Web (never hurts)
  6. Run TuneUp Utilities 2007 and see what they say after the maintainance
  7. Install Firefox again
  8. Tell us if it worked