In Firefox if I click the dropdown on the search field, then click 'Manage Search Engines' I get a dialog in which can be found a link to 'Get more search engines...'.
Clicking that opens an add-ons store tab with the 'Search Tools' filter applied, none of whose above-the-fold suggestions has anything to do with DDG. That's not one-click, it's a runaround.
In contrast, Chrome automatically adds search engines as you browse the web and lets you set any one of them as the default. It also has clearly labelled fields to let you create your own by copying the format of the ones listed. Getting to there takes 3 clicks and a scroll.
Two reasonable gripes: the existence of the free-edit fields is not immediately obvious upon clicking 'Manage search engines...' and neither is the fact that the domain name in the second column is an editable keyword.