How to reset your Firefox default search engine or modify it:

• Open Firefox browser


• Type about:config in location bar and press enter


• Search for configuration key keyword.URL and select it


• Right click on the configuration key keyword.URL and choose Reset from the shortcut menu.
  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=
  
  Otherwise if you like Google’s I’m Feeling Lucky feature you have to write this line:
  
  http://www.google.com/search?ie=UTF-8&oe=UTF-8&sourceid=navclient&gfns=1&q=