Hi, The best way is to uninstall and reinstall firefox.
It will be better if you uninstall it using the free version of revo uninstaller. You have to choose the 4th option, downunder in revounintaller and then first remove firefox by its installer. Wait, then it goes to the other files reside in the registry and other places, if you click next, Just select the bold items shown and click delete. Then if the screen does not closes, just expand the folders to find bold items and then check it for deletion. click next and it will show the residue files, just select all and then click delete. If prompts for the files will be deleted on next boot, click ok to exit. Then reinstall firefox version for your system and reinstall.
I advise this length because, mere deletion of program would not delete the other files. Go to Help tab, click trouble shooting and then select refresh firefox button, If it solves the problem ok. Otherwise do the uninstall and reinstall. If prompted, do not save any personal settings while uninstalling