You're welcome.
Whilst I don't know if that file exists at any given time - I believe that it is created when an email is being formatted if you are using html formatted email. This creation is likely to trigger an avast scan, to do that it is temporarily locked for the scan to happen.
This slight delay to scan is likely to trigger the message from thunderbird about the file and why you wouldn't get a body to the email which presumably is wrapped up in this nsemail.html file.
