What I'd do in your situation, AYG, is as follows:
1. Download a fresh copy of the Firefox installation file to a convenient location on your computer (such as your desktop or 'Downloads' folder) but DON'T run it yet.
2. Open Firefox as usual and export your Firefox bookmarks to an HTML file.
(Bookmarks > Show All Bookmarks > Import and Back Up > Export Bookmarks to HTML. Save your file to anywhere convenient, such as your desktop)
3. Completely uninstall Firefox (via your Control Panel)
4. Run the new installation file, to reinstall Firefox.
5. Open Firefox and import your old bookmarks. (Bookmarks > Show All Bookmarks > Import and Back Up > Import Bookmarks from HTML. Navigate to the location of the bookmarks file you created and double-click on it).
6. Reinstall any extensions you require, as per my post at 2322.