How do I restore it?

You can't in Thunderbird. The status bar message counters have been restored in Betterbird though. https://betterbird.eu/releasenotes/index.html

I eventually found that the hamburger menu, View, Layout, enable the Message List Header shows the total # of messages in that folder, along with the # selected when applicable. Took quite a while to find that....

Thanks for that. Now, anyone know how I might move the total number of messages from the Message List Header back down to the status bar across the bottom, where it used to be? The Message List Header is a waste of real estate to me -- I don't need a large, boldface announcement of which folder I'm in when I have the folder list right next door with the active folder highlighted. I would much rather have the space available for the list of messages.

And since there's plenty of space available in the status bar, why not display the message total there, next to the Today Pane button, where it is smaller and much less obtrusive?

And while I'm at it, which Config Editor parameter would let me change the vertical size of the message pane to whatever I want? In 115.3.3, I'm prevented from making the message pane much bigger than about 55% of the size of the message list pane and message pane combined.

What if I want to just display, say, the most recent three messages in my Inbox and have the great majority of the space available for the messages themselves? Surely there's a way to set this up.