Thunderbird will automatically open the next email when you archive or delete. This can be annoying if you are like me and want to leave some emails sitting in your inbox marked as unread so that you will act upon them later. The following workaround will prevent thunderbird automatically opening the next message and then marking it as read.
Switch Off Message Pane
Go to view -> layout and make sure message pane is unticked. Alternatively, pressing F8 should toggle it on/off.
Change Archive/Delete Behaviour
Go to Edit->Preferences
Ensure the "Close message window..." option is selected .