Okay, auf der Seite des Add-Ons findet sich aber unter anderem folgender Beitrag.
ZitatAfter upgrading Thunderbird to 31, it would no longer let me reply to emails or compose emails, the To field was uneditable. Through a process of elimination I found this addon to be the cause, which is a shame, as it was so good, but until it works with 31 it has to stay disabled and I rate it lower than I would have done previously. Please fix this soon or else I will say goodbye to it forever.