When Firefox performs a search from the Address Bar, it queries the search engine string you have setup within Firefox. If it’s using Yahoo, you can switch to Google. Don’t like Google? Use Ask. Here’s how to switch which search engine Firefox searches when you use the address bar to search.
1. In Firefox type about:config in the address bar and press ENTER.
2. Locate and double-click the entry for keyword.URL
3.  Set the value based on which search provider you would like to use for your address bar searches. Here are a few search strings you can use.

Yahoo: http://search.yahoo.com/search?p=
Ask: http://www.ask.com/web?q=
Google: http://www.google.com/search?&q=


Enjoy...