It has always taken a double click to open a message in a tab or window. If you turn on the Message Pane by pressing F8 you will see the message there when you single click a message header.

Until today I have only had to single click to open an email. I have thunderbird installed on two other computers and am able to open emails with one click.

Then I suspect you have modified the default operation with a 3rd party add-on. As you see in the attached manual it takes a double click to open in a tab or window. http://en.flossmanuals.net/thunderbird/reading-and-organising-mail/

So far as I can remember, there were always double-click to open a mail in a separate tab. Viewing with message pane on (F8) there was and is a single click.