Since Thunderbird was updated to the version 52.2.1, I have issues with the certificates.
I have tried to follow the instructions here:
https://www.avast.com/faq.php?article=AVKB91#idt_0201
But I can not find where to export the Mail Shield certificate with the Avast Mac Security 2016 version 12.7.6dd0a8189365 and a free account.
Any help would be very welcomed.