Thunderbird message preview pane shows only 2 lines since recent update

Since downloading the recent update to Thunderbird, the preview of the message pane is minute, only two lines deep. I can’t read any messages unless I fully open them. The header is enormous. I’ve altered the layout and density so I can at last see the list of messages but the slider that should reduce the list of messages won’t move up to give more space for the preview. I hope this is the right place to post. I’m new and not very tech minded.

Check that you have View > Headers > Normal