Why the changes in the interface and how to fix them

To open a new message I have to right click on it then in opens at the far right of the group of messages that are already open. I want to double click on a message, have the focus go directly to that message and open it.

The way it is working now is a PITA! There no options in the menus to change this behavior.

What has happened to Thunderbird?

What version of Thunderbird?

In 68.10.0 on Linux it would be Preferences > Display > Advanced and set how to open new messages. Options for Windows.

In 78.0.1 it is Preferences/Options > General, scroll down to Reading & Display.

There are also context menu items if you select a message and right click.

If you left-click twice the message is directly opened.