Thank you itman. I'm not a very experienced user and I'm using a German version of the program, so I hope I've understood your instructions correctly. In Eset I see only one root certificate, valid starting 30 November 2020. In Thunderbird's Authorities certificate store I can also see only one certificate, which is valid until 6 June 2028, but with a start date of 9 June 2018. This would be the only one I can see to delete, and I'm not sure what would happen then, so I'm afraid to try it.