See if this article helps. https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER

Am I misunderstanding the link in your reply or does it pertain to websites and not email account security exceptions in Thunderbird?

The link is about anti-virus software messing up things. This goes for both, websites and email servers.

Hi GJGolden, I get this problem too (as shown in your screen shot "add Security Exception". The problem seems to be due to some type of reset function, possibly in Avast AV (or Advanced System Care Rel 10 which I also run), which triggers after a pause of several seconds / one or two minutes after I have "confirmed". If I send or receive a second email promptly, mostly they go through. Although the suggested link above (support.mozilla.org etc) is very interesting, my latest version of Avast does not contain the menu options quoted. It would be helpful if someone was able to explain in simple terms what corrective action I should aim to take, rather than Release specific advice. I am using XP SP3 and Thunderbird Rel 45.5.1. Hope someone can help, Geoff.

It would be helpful if someone was able to explain in simple terms what corrective action I should aim to take,

For any specific question about Avast please ask in an Avast support forum.

I get this problem, too. I use Eset anti-virus software and my ISP is Falcoda. It is a big problem that I do not really understand most of the posts on this. I get the impression that it is probably Eset that is generating a false certificate and some posts have suggested configuring Eset (actually, I think it was Avast) to simply not inspect email traffic. Is this the preferred solution? If so, could someone explain, please, what the implications of doing this are? Would I then lay myself open to incoming emails introducing viruses or malware?

silverhow,

Please post your information to your new thread at https://support.mozilla.org/en-US/questions/1154709