I can't remember if I've just done this on all my browsers a long time ago or what, but if you go to the main web page for duckduckgo.com, and click on the icon that shows up on the bottom of the page, you should be taken to a sort of slideshow about how wonderful DDG is, which should have a big "Add DuckDuckGo to Firefox" on it. That will make it appear as a search provider you can get to via the normal search configuration.