For user who prefer to see a list of
matching websites or search results
instead of auto redirect to a web page
when entering keywords in the
Firefox’s link location bar, it’s
possible to change the Firefox
behavior by modifying its
configuration using steps below.
Enter about:config into the address bar.
Locate keyword.url or type keyword.url into “Filter” text box to search for it.
Double click on the line of “keyword.url”.
By default, the string value for “keyword.url” is “about:neterror?e=query&u=”. To display search results instead, replace the string with one of the following search statements, depending on the search engine you prefer (e.g. Google: http://www.google.com/search?q=)
Click OK when done.
Source & more info