Well you could rename it by removing the .vir so it should be your same file name for the inbox and move it back to the original location. Ensure that TB isn't running when you try that, you would have to pause the Standard Shield when doing this or it is likely to alert again. Your file should be in the C:\Program Files\Alwil Software\Avast4\DATA\moved folder.
I don't know if that would work I don't use thunderbird, as it might be that TB has already created a new inbox.
It is best not to store email in the inbox but use it as a bending tray, read, action and move to a folder more appropriate to its subject. That way the inbox (which is the most succeptable to corruption/deletion) only has a few emails, so any loss is minimised.
The TB F.A.Q has a recommendation along these lines as many AVs would just delete the file as they can't extract the infected email from within the file causing it to be corrupted.
That however won't deal with the problem that you have an infected email in your inbox from some time ago. You could exclude the email folders from on-demand scans to avoid this in the future, more on that when life is more calm.
