Linux Mint ships Firefox with the DuckDuckGo search engine/provider that's used if you type your search terms directly in the address bar (the most convenient way probably).
DuckDuckGo might be good, but it at least looks intolerably ugly and urges a strong desire to remove it.
Here is how to change DuckDuckGo for another search engine from address/URL bar in Firefox on Linux Mint version 12, 13, probably 14 and 15 :) :
type about:config in the address bar of Firefox
promise you'll be careful
in the search bar type keyword.URL
right click on it - modify
replace the string with the DuckDuckGo for another URL that will query another search engine.
If you want to continue supporting Mint, place the Yahoo's URL and use Yahoo search (the example below will use USA Yahoo in English):
Or you can use the dominant and not sharing revenues with Mint Google: