I'm using (and will continue using) Vivaldi Mail and want to see how things work in Thunderbird - this is the first thing that struck me, too. I'm used to collecting all emails coming in from my several email accounts into one unified Unread view, and when I mark an email as read (which I do manually) it disappears from that view and the next one is automatically selected. This allows me to very quickly go through everything that is new, and whatever needs some more work just stays unread until I'm done with it.