Rated 3 out of 5 stars

With FF 3.5, be sure you know what your tabs do by default. Right-click on ANY tab to see a menu, and also notice the IE-like Tab+ box following your farthest-right tab. That IE-like one will disappear if you bump this extension. Just so you know.

This review is for a previous version of the add-on (3.0.3). 

Fixed it and new version available

http://uctb.googlecode.com/files/undo_closed_tabs_button-3.5.0_fx.xpi