When you type something that isn't a Web URL into the Mozilla Firefox address bar, Firefox submits it to Google, then either sends you to the most relevant website or displays a Google search results page if there's no clear match. Firefox uses Google search for the address bar even if you've set the Firefox search box to a different search engine. If you want Firefox to use a different search engine for the address bar, modify the keyword search URL in Firefox's about:config section.
Type "about:config" into the Firefox address bar and press "Enter." Click the "I'll Be Careful, I Promise!" button.
Type "keyword.URL" into the Filter box at the top of the page. Double-click the "keyword.URL" setting.
Type a new search engine URL into the "keyword.URL" box. For example, type "http://www.bing.com/search?q=" to use Bing for the address bar or "http://search.yahoo.com/search?p=" to set Yahoo! as the default Firefox search engine. Click "OK."
Reset the address bar search to Google by right-clicking the "keyword.URL" setting and clicking "Reset."