4.2 DOES work for Thunderbird 5.X Rated 5 out of 5 stars

As Sadi mentioned in previous review, you must change the max version, HOWEVER the APP BUTTON apparently does not work, I cannot find a way to activate it. Hopefully this will be addressed in a future version. Till then you must customize the toolbar and add the compact MENU entry.

This review is for a previous version of the add-on (4.2.0.1-signed).  This user has 2 previous reviews of this add-on.