The following is lifted from Firefox Central:
Add New Search Engines
Firefox Search Box
Up in the top right corner of Firefox, there's a handy search box that puts search engines at your fingertips. When you download Firefox, a number of searches (like Google, Yahoo, and Ebay) are included but you can easily add more. There are literally hundreds of search engines available. The following are some of the most popular:
By default, the Search box lists, with its down arrow, Google, Yahoo, Amazon.com, Creative Commons, Dictionary.com and Ebay. There an additional item: add engines. Whichever one I click on gets its logo put up in the search box.
So if I want to use a particular engine, I click on it in the list and it gets put in the search box. But now how do I use that particular engine?