Yes, it works in Firefox 4 Rated 5 out of 5 stars

The newest version is not yet reviewed but you can find it using the View All Versions link below.

The Tabloc icon is not visible while a tab is pinned as an App Tab, but it does work correctly, and the icon is visible again if the App Tab is unpinned. That's the way App Tabs work.

This review is for a previous version of the add-on (0.4.1).  This user has a previous review of this add-on.