I've been using Thunderbird for several months now and I like it for the most part. I'd like to see a few more themes, but that wouldn't effect it's functionality.
I open my emails in a new tab by double clicking on a message. Then, as I read one I will either leave it or delete it and advance to the "Next" one. When I'm done reading all new messages, instead of the tab closing, it goes back to the one I started on and I have to close the tab manually. I have to determine if that is another new message or the old one I started on.
I would prefer that the messages advance as I advance through them and when I read the last one in line, the tab automatically closes and I am on the last one which I just read and didn't delete. I hope that makes sense.
I used Windows Live Mail for many years and that's the way it worked and to me was a much better method.