De aqui :
Deleting The Custom Search
The XML files for Firefox search engines are located here:
/usr/lib/firefox-addons/searchplugins
Inside this directory you should find a google.xml file which contains the Mint custom search fields. Back this file up and remove it.
sudo rm /usr/lib/firefox-addons/searchplugins/google.xml
ou de aqui
1) Go here http://mycroft.mozdev.org/search-engines.html?name=Google&language=en
2) Scroll or search down to "28. Major Engines" (now it's 29)
3) Go to the first one, Google, click on it and install it. Check the box to start using it right away
4) Click the search drop down list
5) Click Manage search engines...
6) Delete the original Google.