To add DuckDuckGo to Firefox as your preferred search engine, go to that URL. Then pull down the search engine menu in the top right of Firefox, and select DuckDuckGo (SSL). Now, wasn't that painless?
Chromium is a bit more of a pain to set up. For one thing, DuckDuckGo's instructions are wrong. Oops. Go to DuckDuckGo. Right click in the address bar, where the URL is displayed. Select "Edit search engines". Click on "Add". Set it up as follows:
(Note the s in the protocol for https, or SSL/TLS.)
Watch the icons to the right turn into check boxes as Chromium approves your entries. Hit "Save". Select your new entry and click "Make Default" and "Close". Done.
I wonder if Chromium reports back to Google that you've changed your search engine and made the Chocolate Factory just a tad less profitable? Naaahhh, that would be… paranoid.