There's a fairly simple way to restore Firefox to it's default settings using about:config. The Restore Defaults command can be accessed through the menu button on the far right of the about:config page.
Keep in mind, clicking this command with do exactly as it says: it will RESTORE DEFAULTS...in Firefox...ALL OF THEM, including your home page, bookmarks, etc. So be careful with it! Search Firefox Help for detailed information on using about:config. It's a cool tool, but extremely powerful. Good Luck!