Thunderbird Certificate Alert keep popping up

Solution 1:

Port 993 is not HTTPS but IMAPS and thus is not related to fiddler. There are several possibilities why the certificate is not acceptable, like wrong time on the system, SSL intercepting firewalls (often in conpanies), malware... If you need more help please add the details of the certificate to your question, especially the trust path.

Edit: since the details of the certificate were provided the cause of the problem is obvious. From the certificate:

Emitter: CN = avast! Mail Scanner Root O = avast! Mail Scanner OU = generated by avast! antivirus for SSL scanning

This means that this is not the original certificate issued by a trusted CA, but a certificate issued by some SSL intercepting software. I assume, that you have Avast! Antivirus installed and this software will intercept SSL connections to scan the traffic for malware. You need to accept this issuer as a trusted CA - please refer to the manual of your virus scanner for more information.

If you don't have the Avast! Antivirus installed than it might be that a malware claiming to be Avast! intercepts the SSL traffic.