The address bar in Firefox is typically used to enter the correct URL of a website. However, if you type in keywords (not a URL) there, Firefox feeds them to a search engine and displays the results page. By default, the search engine it uses for this search is Google.
Since the address bar can be easily accessed using keyboard shortcuts, this is a quick way to search! And this is just as easy to use as the search bar. The real advantage is that you can remove the search bar while losing none of its functionality and thus save some precious horizontal space in the Firefox toolbar.
Tried with: Firefox 9.0.1