How to remove distro-specific "branding" from Firefox in Linux Mint
From here:
Deleting The Custom Search
The XML files for Firefox search engines are located here:
/usr/lib/firefox-addons/searchpluginsInside 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
or from here
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.