I am a former user of both Mozilla Thunderbird as the Avast. Still I did not adapt to the webmail idea, I prefer to have software to manage my emails the way I have always done since the days when time connection to the Internet at home was something impossible. And Avast also been with me for several years, it never felt the need to pay for antivirus, and free think he is the best.
But a problem that has been going on for some time is that Avast wants to scan my IMAP connections to Gmail by default, but the connection is SSL he needs some additional steps with certificates that I never wanted to give me to work. Previously appeared a warning from Avast when the first connection attempt happened and I just desabilitava the option of scanning SSL connections. But now the warning is no longer appearing and in recent days had some problems with Thunderbird not lowered any email but gave no error.
The solution is the same as always: abrir a interface do Avast -> Configurações -> Proteção Ativa ->Módulo de E-mail -> Configurações -> Escaneamento SSL e desmarcar a opção “Escanear conexões SSL”.
Of course, this eliminates the protection of Avast in emails, but at least the Thunderbird back to work without major problems.