It does what it says. But when I tested I have to add "http://grooveshark.com/*" to include sites in order to "widens usable area" works.

I'm using Firefox Aurora 5.0a2.

Just as the description, removed ad bar on left, expanded the usable area, and removed the obnoxious drop-down MSN advertisement. This is on firefox 4.0b13pre (minefield). Thanks!