I'm betraying my ignorance here. Having done a little exploring of the subjects of emails and scripts, it appears that emails do not carry scripts, unlike web pages. However, I was directed by Gmail a few years ago to turn on scripts in Thunderbird because Gmail needed them, seemingly as part of communication with Gmail's servers. Thunderbird's Advanced Settings contains numerous Javascript options including 'javascript.enabled' which can be set to true or false. Clearly javascript is used but I am not clear what for, other than the Gmail requirement.
I an guessing that the actual email body contents do not include javascript. I shell keep on digging to learn more or else give up in despair. Malwarebytes Anti-Exploit is equipped to detect bad email scripts and attachments. I assume that Avast Mail Shield protects against bad attachments but apparently not bad scripts.
While was typing the above, Avast One was dumped on me without a by-your-leave.