I think that you need to do a complete uninstall and reinstall of Firefox. You should uninstall it using
Revo Uninstaller (there is a free version). Uninstall using Advanced mode. This will wipe your user profile, including you bookmarks. You can save your entire profile (including bookmarks) and restore it using
MozBackup. Remember to save it somewhere outside Firefoxe's directories. Alternatively, you can use Firefox Sync to save and reload your bookmarks.
You can trust Revo Uninstaller to select the correct registry entries and files to delete after the Firefox uninstallion process has completed. I always uninstall applications using it and have done so hundreds of times. It has only let me down once and that was before I switched to Windows 7.
Remember to have some other way of accessing t'internet while firefox is unavailable.
Faced with the choice between changing one�s mind and proving that there is no need to do so,
almost everyone gets busy on the proof. -- J.K. Galbraith