Source: http://www.labnol.org/software/browsers/interchange-firefox-address-bar-and-search-box/3919/
The following screencast animations shows how to exchange the position of Firefox Location Bar with the search box as in the screen capture above.
Idea by Cameron. Do check this guide on how to prevent Firefox 3 from showing bookmarks in the address bar.
Interchange the Position of Firefox Address Bar & Google Search Box - Digital Inspiration by Amit Agarwal