The drop-down tells you the price difference between the page you're on and the lowest price it could find, and offers a link to that retailer and other sites' prices in right-hand buttons. That's all it does, and it doesn't seem to slow down page rendering to do so. Price isn't everything online, of course, and the add-on doesn't factor in average shipping costs, but serious shoppers won't mind checking out a competitor if a real savings is in play.
Invisible Hand is a free download, works wherever Firefox does.
Invisible Hand [Mozilla Add-Ons via The Red Ferret Journal]