This article tells how to export the CA using the Windows version of Avast:
https://www.avast.com/faq.php?article=AVKB91#artTitleThe Mac version has an entirely different UI from what is described there. I can find no place in the Avast Mac UI that allows me to export the CA. But Thunderbird is now requiring it.
How do I export the Avast CA on a Mac?