How to reset your Firefox default search engine back to Google:

1. Open Firefox
2. Type about:config in location bar and press enter
3. Search for configuration key keyword.URL and select it
4. Right click on the configuration key keyword.URL and choose Reset from the shortcut menu.
5. If clicking on “reset” nothing happens just write this string:

http://www.google.com/webhp?hl=en#hl=en&cp=4&gs_id=i&xhr=t&q=

That’s all, folks!