Date: 2015jun30 Updated: 2019jan11 Product: Thunderbird Q. My "Confirm Security Exception" button (for certificates) doesn't work. How can I fix this? A. For me, removing the cert8.db and cert_override.txt files in the profile folder did the trick. Location of your profile: http://davekb.com/browse_computer_tips:thunderbird_where_is_my_data:txt I read that solution here http://forums.mozillazine.org/viewtopic.php?f=38&t=1160375 I'm pretty sure its caused by my virus-checker intercepting the TLS (SSL) connection to the IMAP servers I connect to. If I disabled that function in the virus-checker it would no longer perform this man-in-the-middle function. For mail on my LAN I could also switch to not using TLS - then there would be no certificates.