this happens to us often

i believe it is due to our domains being hosted in such a way that our certificate is in our host/ISP's name. i.e. our domain name does not equal our host's security certificate name <we have been with them for 15 years!>

we get a round this, about twice a year, as follows: thunderbird kicks up warning dialog like yours + another dialog box pops up below the main thunderbird window where we have to "allow" the exception.

for us, it pops up both on send + receive, + once we have confirmed it the system is good for the next 6 months to a year... but you have to do it on every computer with thunderbird + that host

The prompt to add an exception for self-signed certificates is broken in TB 78:

https://support.mozilla.org/en-US/questions/1308004

Looks like the fix should appear in 78.4, possibly by next week.