A Quick Thunderbird Question

Something has changed in the way my Thunderbird email program operates, but I can’t figure out how it has changed. Originally when a new message arrived I would single click onto that message and it would open in the lower pane and the message would be shown as “read”. Now I have to double-click the message which then opens in a separate window. If I only single-click on the message it will not open and it remains as “unread”.

I have looked at all the tools and options to change back to the original format but cannot see where to do this. Have I missed something obvious?

Go to Tools -> Options -> Advanced -> General and there is a check box labeled
Wait 1 seconds before marking a message as read
where you can select the number of seconds. On mine (Ver, I have that checked. I hink thi is the relevant setting.