Re: Thunderbird can't connect to Dovecot (bad certificate: SSL alert number 42)

cert had an invalid/incorrect hostname

fyi,

https://kb.mozillazine.org/Files_and_folders_in_the_profile_-_Thunderbird
	...
	cert_override.txt
		This is an optional file used to store a security exception. It appears to store the host name , thus preventing you from creating a security exception for a rotating SMTP server.
	...

for ref,

Firefox: How to audit & reset the list of trusted servers/CAs
 https://access.redhat.com/solutions/1549043