In the message pane, there is a DELETE button. If you push it, the current message is deleted and the next message is displayed. This is ok when you want to delete one message after the other. I would suggest to add a NEXT button. This would be a way to browse thru messages, deleting some and leaving others. In the current version of Thunderbird you have to doubleclick the next message in the list pane in order to view the next message.