Change Firefox default search engine behaviour in the location bar

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