It started with wanting to use one of the themes I used before updating to 91, which I could not select. Followed by trying to install a theme from the official Thunderbird add-ons webpage and getting the next message: "thunderbird has prevented this site from installing an unverified add-on" It doesn't matter what theme or add-on I try to install, I get the same message. Does anyone know what is going on? Is there a solution? Thanks!