Thunderbird (sorry I don't use it) has a different method of storing emails and that can and does cause some problems
Sorry David, I do use Thunderbird and I am very familiar with its internals and workings and your comment is completely without foundation.
Base64 encoding is not about encryption or security. All (well almost all) POP3 non-text email attachments are base64 encoded in billions of emails around the world every day. Base64 encoding is what makes it possible to make a binary file attachment look like regular numbers and letters and able to send them through the old as dirt SMTP protocol that was really only designed to transmit text. Avast knows all about base64, it caches the attachments, decodes them to turn them back into the real files and thoroughly scans the real files just as it would any other file.
I recently spent quite some time sending every available eicar virus format I could find through to Thunderbird (1.5.0.4) ... and avast caught every one of them with the IM scanner setting at normal.
Getting back to my virus, it look's like it's scanning but not getting the very basic test to me.
Can you tell us how you know it is being scanned - are you seeing the number of messages scanned in the Internet Mail scanner increasing? Are you seeing the subject line of the eicar message recorded in the scanner?
I am just wondering how you are getting the eicar message delivered to Thunderbird at all when most major ISPs and mailing services include virus scans that will prevent even the eicar virus from being delivered.
Could it be that the connection you used to deliver the eicar message from the mail server was a secure session? Those cannot, by definition, be intercepted by avast to scan the messages and, if not stopped at the mail server, will be delivered to the Thunderbird messages store (or that of any other mail client for that matter).
Last but not least, I suspect this is a very short mail message. Could you capture the view of the message source in Thunderbird (select message then View > Message source) ... obscure any personal details of yours and then post the result here, if not all of it then at least the message headers?