It's very good while running on Thunderbird below V60. Here the sync with a synology works without problems.
After Tunderbird V60 it's broken. Checking the carddav-server fails whther you do it with https://IP:Port or https://IP:Port/addressbooks/users/$USER/addressbook or https://IP:Port/addressbooks/users/$USER/addressbook/. The tip to set the cookie variable from true to false also doesn't work here.
This review is for a previous version of the add-on (31.2).