Specifically this is for the search done in the Location bar.  Setting that little search bar is a no brainer and most people can figure it out.  The location bar is a different matter.

From FireFox’s support site about the location bar search.

1. In the Location bar, type about:configand press Enter.
   • The about:config “This might void your warranty!” warning page may appear. Click I’ll be careful, I promise!, to continue to the about:config page.

….
….
….

Changing the keyword service

You can specify any valid URL to perform the Internet Keyword searches by modifying the keyword.URL preference in the about:config page:

• To use the Browse by Name service from in earlier versions of Firefox, keyword.URL can be set to http://www.google.com/search?ie=UTF-8&sourceid=navclient&gfns=1&q=. Google will forward you to the relevant site if it finds a clear match. Otherwise, the search results page will be displayed.
• To use Bing, set keyword.URL to http://www.bing.com/search?q=.
• To reset the Internet Keywords service to use a standard Google search, right-click on keyword.URL and select Reset.

There is a lot more on that website, but this is the bit that pertains to my topic.