Mozilla added DuckDuckGo for everyone themselves along with a recent update
The first thing we’ve done to give our users more control is to add DuckDuckGo as a pre-installed search option in Firefox for Windows, Mac, Linux and Android. Search is an integral, and deeply personal, part of online life. The things you search for can say a lot about who you are. DuckDuckGo gives you search results without tracking who you are or what you search for. Other engines may use tracking to enhance your search results, but we believe that’s a choice you should get to make for yourself.
-- https://blog.mozilla.org/blog/2014/11/10/celebrating-10-years-of-firefox-2/