I usually like to have 4 emails visible in the upper right window and view emails in the lower right 'preview' window. The most recent update limited the size of the preview window, so I can see lots of emails in the upper right window and only an inch or so in the preview window. I prefer to see fewer headers in the upper right window and more 'preview' in the lower right window.

I like Thunderbird and have been using it for many years. This latest update 'fixed' something that didn't need 'fixing'. Thank you.