By default Firefox 4 uses a Google search to help you find any mistyped URL locations or addresses placed into the location bar. Somehow I ended up with Yahoo as the search in my location bar and Firefox doesn't give you an easy option to change it back.
I assume some program that I installed hijacked my location bar search based on this Mozilla support article: http://support.mozilla.com/ast/questions/767962#answer-129005. Fear not, it's easy to change.
To fix the problem you need to edit the Keyword.URL field in the about:config section of Firefox and this article here has the steps on how to do that.