Looks like I'm affected with the same issue.
The problem started to appear around 12:00, now I have the same on two computers both running ESET and Thunderbird 78.5.0.
Symptoms on my side:
In Thunderbird IMAP accounts cannot download new emails, they are stuck with "Checking mail server capabilities" message in the footer.
Sending emails is possible via SMTP, but after message is sent, Thunderbird displays popup that outgoing message cannot be saved to "Sent" folder.
The workaround for me is to disable SSL/TLS filtering like here: https://support.eset.com/en/kb3126-disable-ssl-filtering-in-eset-windows-products
Z.