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:
https://duckduckgo.com/?q=

Use Google, with "I feel lucky" effect, going directly to the best matching result:
http://www.google.dk/search?ie=UTF-8&oe=UTF-8&sourceid=navclient&gfns=1&q=

Just go to the Google result page: