It allows you to make, if you want, a search using all the search engine together, opening the results on separated tabs. You can also decide where to show the search's results: on the same tab, on a new tab (default) or on a new window. I've also included "Feeling-Lucky" search which returns you the first google result (use the "Show" item).
Try the new version fully Firefox 4.0-compatibile! DOWNLOAD it there: https://addons.mozilla.org/firefox/addon/11173/contribute/roadblock/?src=version-history&version=2.0.20110408 :)