See https://support.mozilla.org/en-US/kb/message-threading-thunderbird

Firstly press Alt-v and click "Sort by" on the drop-down menu that appears. Towards the bottom of that are "Threaded" and "Unthreaded." Click "Unthreaded."

That will sort out the folder you are currently displaying but only that one.

To make it work in every folder right-click somewhere on the line with the column headings and choose "Apply current view to folder and its children" at the bottom, then pick one of your accounts, then the top of the next list that comes up. See picture as hopefully that'll make it clearer.

You have to do it for every account you have.

To get rid of the preview pane just press F8.

Thank you for your reply. I followed your instructions to the letter. Or as near as I could. It was not possible for all my accounts becuase, as stated previously, four of them keep "disappearing". I can see those accounts' emails in the unified view, but cannot access them individually unless I close and reopen TB. Then I have a short (few seconds) window of opportunity.

In any case, when I reopen TB, every account where I selected unthreaded has reset itself back to threaded.

In short I appreciate your assistance, but can I try something else? Your solution did not work.

Is there a way to completely uninstall the threaded "feature"?