Try a clean reinstall:
1. Download the Firefox installer from Mozilla.com
and save it to your desktop or other location.
2. Make sure that Firefox is closed completely
3. Delete the Firefox installation directory (On Windows, this is typically the C:\Program Files\Mozilla Firefox folder that contains "firefox.exe" and other Firefox program files and folders).
4. Reinstall Firefox by running the installer you downloaded previously.
Your personal settings, including bookmarks and history, will not be lost.