This is nuts. Mozilla should at least allow the user to set a custom search engine using an about:config option, like the old keyword.URL [1] option, which was removed after Firefox 23. I use DuckDuckGo with several customizations as POST parameters and it would be very cumbersome if I have to write an extension to use DDG the way I want it.
[1] http://kb.mozillazine.org/Keyword.url