You need to highlight each folder and click View>sort by>unthreaded. If this applies to just a few folders, just repeat the process as necessary. If you have many folders or plan to add many new folders, you may wish a different approach, below:

More info: To change default for new folders, In config editor, you can change default that applies to NEW folders you set up: mailnews.default_view_flags 0=unthreaded

Config editor is available at Tools>settings>general and scroll to bottom right.

To set many existing folders, follow these steps:

  In the Message List view, in the column header on the far right, open the button that shows context menu.
  At the very bottom is the option:
  -> "Apply current view to..."
  -> "Folder and its children..."
  -> Hover over an account
  -> In the next menu don't select a folder. Instead select the account name itself.

This isn't a perfect solution, but it does address the issue of unthreading many folders.

I very much appreciate your detailed answer, and I am sure it is correct as far as it goes, but I just don't have the patience to think about all this today.

I already have roughly 100 mail folders and I have already gone to the trouble of changing them all so that they all have the same consistent and reasonable set of columns. I do create new folders often, since I use a new folder (not a thread) for each significant conversation.

I think I am lucky to have Thunderbird at all, since it almost died a few years ago due to neglect. It has been worked on a lot and is probably one of the best email clients in existence. But it does have a few flaws, and to me the whole idea of conversation threads is one of them.

I'll consider these solutions again in the future. Or maybe I'll just wait until someday someone gives us a way to choose threaded or unthreaded views for all existing and new folders. Again, I thank you for providing what is probably the best set of solutions given the current design.

There is a fork of Thunderbird, called Betterbird. It defaults to NOT threading messages. And thank you for your kind words.