You have a problem with your antivirus is my guess and do not, and probably did not have, any problems in Thunderbird that you could fix but uninstalling it.

Most of those posting this week with these sorts of issues are Norton customers. Are you? You might want to apply the fix for your issue they have if you are.

Also it claims there is a potential security threat with Thunderbird's own web addresses: "live.thunderbird.net." and "addons.thunderbird.net.".

This error is a strong indicator your antivirus product is the root cause and has changed the self signed certificates they use. Thunderbird does not automatically trust self signed certificates. It must be told they are Ok, by the user who can make an informed decision that they trust the issuer shown in the certificate

Thank you Matt, I had been a loyal customer of Norton since before Windows 3.1 - way back in the 1980's. However, uninstalling Norton 360 has solved all my Thunderbird problems. I can now visit thunderbird sites from thunderbird and see images in incoming emails. I will be cancelling my Norton subscriptions that are about to automatically renew.

Thank you Matt, I had been a loyal customer of Norton since before Windows 3.1 - way back in the 1980's. However, uninstalling Norton 360 has solved all my Thunderbird problems. I can now visit thunderbird sites from thunderbird and see images in incoming emails. I will be cancelling my Norton subscriptions that are about to automatically renew.

I was a Nortons customer until Symantec bought the product from Peter Norton really and the bloat made it more a resource stealer. I have cycled through most of the major antivirus programs at some point over the years since then and for the last couple of years have settled with Windows Defender. It asks little, has prevented(I assume) infection in that time and I have saved a lot of money on a five seat license I was paying for previously from ESET