Not knowing what way thunderbird stores emails, I can’t really offer any further information.

I do know that if avast picked up potential virus in an OE .dbx file (lets say inbox.dbx), it won’t allow you the option to delete or repair as it can’t/doesn’t know how to extract individual emails from within the .dbx file. You have to effectively find the offending email within your email program and delete it manually.

So I can only assume that this wolud be the same if TB used a similar method of storage (multiple emails compressed in one database file).

In theory avast should have picked up this email virus before it got into your inbox (if it was able to pick it up in a regular scan), or it may have been there before you installed avast or it was a new virus, recently added to the iAVS.

I appreciate that this doesn’t resolve your problem of getting back you old inbox, that information would best be sought at the TB forum.