I can't speak for the other person, but I'm not using any add-ons of any sort (other than a theme), and no antivirus with any real time scanning of any sort either.Also worth noting that I did post in the support forum: https://support.mozilla.org/en...
I also have this happen. I started using Thunderbird around version 78, and it's always been like this. I can indeed confirm that all messages in all folders and mailboxes are marked read instantly upon being opened instead of after my setting 10 sec...
