As I explained in the other post yesterday, avast! scans only the first message in the mailbox, as it looks like an ordinary EML file. So, if this one contains a malware, then it is detected and the subsequent action may confuse Thunderbird. The rest of the messages are not scanned (well, in special cases a malware may be found in the full scan of the file if the infected object is stored in plaintext there - but I believe it's quite rare).