Once you install it, this extesion will be added in the Facebook toolbar. We'll see a warning featuring the number of friends who have decided not to be your friend now. If you click it, you'll se their names.
From the on, they'll be your 'unfriends' and you'll be able to acces their profiles from Facebook toolbar.
Unfriend Finder is a curious application that allows you to know who your friends really are.
This is the extension for Google Chrome, but you can find Unfriend Finder for Firefox in the Official Website.