Rated 4 out of 5 stars

As Responded by jonmyers, Enable cookies in Thunderbird and not the browser. Works just fine now. If Philipp the Developer added this note in the Add-ons description a lot of complaints would not be written.

This review is for a previous version of the add-on (3.1). 

I'd much rather fix the cookie bug by properly making an exception or enabling cookies only for the login dialog. I'll see if I can get to this soon though. Thanks for the feedback!