I prefer this as well. Here's how I did it: 1. Type "about:config" in the URL bar. 2. Click "I'll be careful, I promise!"3. Type "keyword.URL" in the filter field. This will show you all config entries with "keyword.URL" in it. There should only be one.
4. Double click on keyword.URL 5. Replace what's there with: https://duckduckgo.com/?q=
That should do it.
