Can't get rid of "Add Security Exception" pop-up

Sometime ago, when I was regularly using more than one computer, I thought I'd found a way to have my Thunderbird address book centrally located and accessible from both computers. I got no success from attempting this, but have suffered some side-effects every since. Configuring Thunderbird back then somehow involved using the location "demo.sogo.nu" and I believe it related to a security certificate. What used to happen was, soon after rebooting the computer, I'd get a couple of pop-up messages about connecting to demo.sogo.nu that I could clear away easily. Recently, something changed regarding this certificate and I now get pop-ups (screenshot attached) that continually bother me to create a security exception. I want these gone! I've gone into the Certificate Manager and can't find anything under the Server tab to delete. One other clue is a second screenshot that I discovered somewhere in Thunderbird that actually shows the *.sogo.nu certificate - but offers no way to delete it! Can anyone give me a hand? My only alternative is to literally annihilate Thunderbird and start from scratch, but I really don't want to go there.