If you enter a value into Firefox's location bar that does not resolve as a hostname, Firefox automatically issues a Google search.
Up until Firefox 3.6, the default behavior was to execute a I'm Feeling Lucky search, i.e., to take you straight to the most likely search result for the value you entered. In Firefox 4, this behavior was changed to issue a regular Google search instead. If you have become used to the previous behavior and want to change it back, this add-on will do exactly that.
For the tech-savvy: This add-on sets the keyword.URL preference to what Firefox 3 uses, and resets it to the default if you disable or uninstall this add-on.