First please note that I am just an avast user - I do not speak for the avast team.
I am sorry to say that none of the on demand avast scanners available for the avast Home edition has any idea that a file being scanned by it is part of Thunderbird email.
One big part of the problem is Thunderbird itself. Many email products (like Outlook and Lotus Notes store the mail files in an encrypted format that prevents anyone just reading the email contents). Those products not only encrypt the mail content but also any virus information that may be contained in the emails, so that an antivirus scanner, like avast, cannot detect it.
Thunderbird stores the email in each folder in clear text, but each folder is just a clear text file with a filename such as Inbox or Sent or a folder name that you have chosen. That means that if an email in one of the Thunderbird folders is recognized as a malicious script, for example, then avast will identify the folder as infected and move it completely to the virus chest.
Avast has no way of knowing that this is an email file and still less has it any way of identifying a single infected mail message in that folder and surgically removing it from the folder.
That is why it is vital with Thunderbird to use the Internet Mail scanner, which does understand the format of email, to scan email as it is being received (or sent) and to prevent any infected email messages from ever being put into the Thunderbird mail store.
If you can identify the infected message in your Thunderbird folder then it is imperative that you:
1) delete the infected message from the folder
2) empty the trash folder into which the infected message is moved
3) compact the folder from which you deleted the message
4) compact the trash folder to which you emptied in (2) above
Remember that in Thunderbird a mail message is only ever really deleted when you compact any folder in which it was held or any trash folder to which it was copied.
I realize that this will not solve your problem but I hope that it helps.
