Thanks for your response. I can get as far as exporting the certificate from Avast (and saving it to my hard drive), but I can't import it into Thunderbird, because I can't close the "Add Security Exception" popup.
And if I can't close it, I can't access Thunderbird. I've tried clicking on every option in the "Add Security Exception" popup, but I can't get it to close.
As long as that popup is sitting on my Thunderbird screen, I can't do anything in Thunderbird.
It's entirely possible that I'm making some stupid mistake (repeatedly), but I've tried the procedure several times and can't get it to work.