New Thunderbird changed my display layout, against my desires

Up until yesterday (2025/07/18) My display was set up with two columns: the left column was a hierarchy of folders and sub-folders; the right column had two sections: a list of messages (subject, size, from, date) in its upper section; and the text of the currently-selected message in its lower section. With this update (2025/07/19) my display is divided into three columns. The left column is still a hierarchy of folders. The center column is a list of messages. The right column is the text of the currently-selected message. This is awkward: I must choose between making the center column too thin to show all the message metadata (subject, size, from, date) or making the right column too thin to easily read the message text. How do I restore the text-below-messages layout that I want? Thank you.