From the Message List Pane, selecting a message now displays the message below in the Message Header Pane and also opens the message in a new tab or new window, depending what I setup.
These popped up windows are out of control. I can clear 70% of my mail just by deleting the msg from the Header Message Pane. I have no need for these addtional popups which seem to stay open and unseen until I minimize the Thunderbird window when suddenly I have to close each new popup.
Changing the account setting under "Reading and Display" does nothing.