You don’t have to use Stunnel and OpenSSL, etc. if you are using avast 6.0 as it handles the secure connection. This was also the case with avast 5.1.889, I don’t know why you haven’t updated to avast 6.0.

But you need to disable the SSL in the thunderbird email account settings.

Check this article, although this relates to avast 5.x it is also relevant for avast 6.0 - avast! 5.x: SSL Problems The Mail Shield may not scan some e-mails - see http://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=458.