Rated 4 out of 5 stars
I have been using this extension for at least 2 years with success. Unfortunately, few days ago, as soon as I installed TB version 52.0 (final release), I got a problem caused by gContactSync both using its "official" 2.0.11 version and also its developmental version 3.00alpha4.
The problem is that I can't drag and drop anymore contacts from an addressbook into mailing list(s) both for the "GMail synced addresssbook" and also for all the other addressbooks I have.
So I had to inactivate gContactSync at the moment, unless when I need to sync my Google Contacts otherwise I cannot assign contacts to mailing list(s) and/or I need to move contacts from an addressbook to another one.
I hope that it could be possible to patch this annoying bug.
I uploaded versions 2.0.12 and 3.0.0a5 with a fix for Thunderbird 52.0. Thanks for letting me know and providing a translation!