In another thread (unrelated) I saw a mention of the ‘repair option’. I tried it and it seems to have fixed my problem. Make sure to reboot after…I didn’t right away and thunderbird was not working (I kept getting an ‘add security exception’ popup).

The comment on the other thread:

have you tried avast repair controlpanel > ad/remove programs > avast > uninstall/change > repair option ...wait a minute and reboot