The old version of Thunderbird had four panes: Files on the left,; Sender and Subject on the upper right; and messages in the lower right quadrant.

I count that as three panes. What was the fourth?

The fourth pane was the "folder" pane on the left. While that "pane" may not affect you as a programmer, it is a "pane" for the user, like me. My primary question, or challenge, is to go back to the "classic" view, not that aberration imposed under the NTFS system. The old version worked in Windows 10, why can't we still get it?

What you are trying to explain is that you have the view option in Thunderbird set to "Vertical View" as that has never been a default view, at some point you must have managed to find the setting to change it.

≡ > View > layout > Vertical.

As you apparently want to see the message as well you will need to ensure that the message pane is enabled on that menu, or press F8 to toggle it.

Now to dispel some misconceptions you apparently have. This is a peer forum. That is those that respond to you will be fellow Thunderbird users. Paid staff do not come to this forum to offer user support or to look for user pain points. There is an occasional appearance, but your issue is not the reason they are here. What you almost certainly will never see here is a developer. I recall once in the past 12 years, so if it happened for you it would truly be a standout.

Making threats about continued use is not going to get any weight in this forum as you are threatening fellow users that you will not longer use the product. I see such gratuitous claims often in this forum. Perhaps it is a normal part of seeking support in some nations and some cultural environments. It is does nothing positive for your case in this forum.

Look at it from my perspective. Do I really care what you use to manage your email, or even it you do use email. I use Thunderbird and volunteer some time to help other users when I see fit. I get no bonus for putting up with attitude or smoothing ruffled feathers. Nor do I get anything out of regurgitating what is already written in the user support articles see the support document you apparently missed here https://support.mozilla.org/en-US/kb/how-change-thunderbird-layout

Nothing in the Thunderbird view layout has changed substantially since version 2. A couple of releases ago there was a card view introduced, I find it less than useful, so I continue to use the same list/table view I have used since V2. Likewise I despise the layout you are seeking and have always used the wide view. Even with modern widescreen monitors I see no reason to try and shoehorn the message into yet another column across the screen. Microsoft came up with the idea in outlook, but I still prefer to devote the full width of the monitor to the email I am reading. In a similar vein I still have the menu bar enabled because none of the new "intuitive" ways of doing things is anything like as discoverable as using a menu that lists everything.