Assegnate 3 su 5 stelle

Very useful, but buggy. Like user SternByName wrote, header line disappers after click on plus button. Only restart of TB gives header line again. TB 3.0 (BuildID 20091204171430)

Questa recensione riguarda una versione precedente (1.1.4) del componente aggiuntivo. 

I have heard several bug reports of this kind. But I was never able to reproduce this problem. Could please post some more informations at the thread at mozillazine:
http://forums.mozillazine.org/viewtopic.php?f=29&t=1405155

Is this the first time, you installed this add-on? What other add-ons have you installed? Are there any error message in the error console (Tools->Error Console)? Does it work after the first restart of Thunderbird or does the error persist?