If you type some keywords in the address bar (also known as location bar) in Firefox and press enter, it will perform a search for these words.
To change search engine, or configure the behaviour, type about:config in the location bar, and search for keyword.URL, update to match your needs.
To use the DuckDuckGo search engine:
Use Google, with "I feel lucky" effect, going directly to the best matching result:
Just go to the Google result page: