Rated 5 out of 5 stars

I have to revise my rating: give now 5 stars.
The tip from stroehli did the trick. Javasript.enabled was set to 'false' (maybe by default or by another Add-on). I set it to 'true' now everthing works fine (Thunderbird 6.0).
For German users:
Einstellungen->Erweitert->Allgemein->Konfiguration bearbeiten...

This review is for a previous version of the add-on (4.4.1).  This user has other reviews of this add-on.