sigh Thunderbird has so many great features under the hood, but the hood is the biggest issue I have that prevents me from switching full time. Classic and wide layouts are not optimized for wide resolutions in modern laptops. Their vertical layout is unusable. The columns in the list of messages are horizontal, taking up too much width. Each layout is a tradeoff on whether I want to message pane to be usable or the list of messages to be usable.
I was never satisfied with their layout either. Always liked the way it behaved though. Ended up switching to Postbox, a Thunderbird fork, about 7 years ago and haven't really looked back. You have to pay for it, but it's a pretty solid email experience for what you pay (I think I paid $40 for a lifetime license a few years ago).
Have you tried turning off some of the columns in the message list and using Vertical view? There's an absurd number of them that you really don't need all the time.
I have, but the biggest horizontal space eaters in that view are the most important ones - subject, from, and date. Turning off the others don't really help that much. Besides, Mail.app gives me the all the same information as in Thunderbird's, but the list of messages is rearranged for that view. It doesn't eat into the message plane. Thunderbird's vertical layout is just a re-arrangement of the panes.
Edit: I mixed Vertical and Wide in my original message. I meant the vertical view is unusable. I've edited it.
Each to their own I guess. I use vertical and like it. Left pane on the left is all my accounts and folder tree; middle pane is email star:subject:sender:date; right pane is email contents.
Above the three panes I simply have the message filter box and above that is the main toolbar (get messages, write, address book, view drop down, quick filter button).
Simple but works well for me on 1920x1080 or higher (and dealing with about a hundred or so emails a day).
