Have you ever noticed that even if you change your search engine in the upper right search box, searches from the URL bar (using ‘?’ or multiple words) still use Google? In older versions of Firefox, you had to mess with the keyword.URL preference to change this, but things are much easier now.
Go to about:config and search on browser.search.defaultenginename. This is the preference that controls your default search engine. Change it to the name of the engine you want to use and you’ve changed keyword search.
UPDATE: Bonus! This also changes the search engine used for about:home.