Download the free trial version of Revo Pro. (http://www.revouninstaller.com/).
One installed you will see and option for forced uninstall. Type in Firefox as the programs name and it will find any left over files. Alos you can reinstall Firefox and then do an uninstall and select advanced scan. Look at each registry entry it finds and let it delete those you see to be tied to firefox. Firefox is usually very good about uninstalling itself.