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:
http://www.google.dk/search?ie=UTF-8&oe=UTF-8&sourceid=navclient&q=
To disable the search from location bar, search for keyword.enabled and double-click on it to set its value to 'false'.
From http://support.mozilla.org/en-US/kb/Location%20bar%20search
