Certificate accept dialog does not show on taskbar, does not appear in Alt-Tab list
First of all, Thunderbird for whatever reason does not trust Lets Encrypt certificates. Is Thunderbird special?
Next, when it asks to confirm the cert, the confirmation dialog and a message box appear, hidden under the new email window. Is there a good reason why they should not pop up, like all normal new dialogs and message boxes pop up, in millions of other softwares? Does Thunderbird have to be special and hide its dialogs?
And lastly, it takes a deliberate development effort to hide new dialogs. Is there a good reason for you to do that to us, so that we have a hard time Alt-Tabbing into the confirmation dialog? What is it?
ప్రత్యుత్తరాలన్నీ (1)
I think you will find that certificate is reported as expired, not that the issuer is not trusted.
Yes there are issues with certificates and the way errors bubble to the top when they are issued by the Mozilla platform layer. It has been going on for some time, and will probably continue for some time more.