1)Open Firefox in Safe Mode, from your programs menu or by typing “firefox -safe-mode†in Start->Run.
2)Choose what you want to be brought back to its original settings by checking one of the following options:
Disable all add-ons:
Disables all added themes and extensions.
Reset toolbars and controls:
Removes any made toolbar customization.
Reset bookmarks to Firefox defaults: Replaces your bookmarks with the default bookmarks.
Reset all user preferences to Firefox defaults:
Restores the default Options and preference settings and resets the default theme. It also changes back to default all the entries in the about:config page.
Restore default search engines:
Adds back all of the default search engines (Google, Yahoo, eBay, Creative Commons, Answers.com, and Amazon.com) , without removing any added search engines.