Here is quick guide of "How to restore Firefox default setting"
1. From bottom status bar, click Start > Run and put " firefox.exe -safe-mode "
2. You might get error as below " Windows cannot find ‘firefox.exe’. Make sure your typed the ……"
3. Then open a DOS window, and go to the specific directory ( c:\Program files\Mozilla Firefox\ )
and type " firefox.exe -safe-mode "
4. Once below window popped up, choose options you want to remove. Careful, if you don’t want to loose your bookmark, make it backup.