Actually, there is just one approach and it does work:

This is the official (to my knowledge) documentation. Well worth the read: https://support.mozilla.org/en-US/kb/message-threading-thunderbird

Here is my own documentation on threading, duplicating the above, but it may help: 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.

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To set many *existing* folders, follow these steps: (I creates a crude video that walks through the steps. The video was created with table view for the message list pane.

   https://davidkirk.org/SettingDefaultFolderView.mp4  that demonstrates the following steps:

   First, highlight a folder that you have set to unthreaded	
   In the Message List view, in the column header, on the far right, open the options 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.

ALSO, for future installations of Thunderbird, 
- create a simple text file with name of user.js with these lines: 
     // Set default threading to UNthreaded  ###   <== this line is optional, just for documentation
     user_pref("mailnews.default_view_flags", 0);
- install Thunderbird and exit prior to setting up accounts	 
- copy the user.js file into the profile
- now, when you start Thunderbird, unthreading will be the default. Problem solved. 

I need to get rid of the threading thunderbird placed on the e-mail subjects. Not column thread toggles. "Thread" is a word used in two ways.

Are you using any addons that focus on conversations?

David, I just tried to do what you suggest, and when I go to Config editor there is no "mailnews."

Thunderbird folks, if you are listening, based on the number of threads I see about this problem, MANY people hate threading, and want it gone. Gmail has a simple option to disable threading. Why doesn't Thunderbird?

I regret that you are having difficulties. Please recheck in config editor, as there are more than a dozen entries beginning with 'mailnews' - I understand your dislike of threading, as I also prefer unthreading, but it seems this ship has sailed. I created the above-mentioned user.js and I copy it into the profile after installing thunderbird and it becomes a non-issue.

Hi David!

Ok, I looked again, and I was looking at mail.news, not mailnews, so I have now found it! Thanks so much for your kind and patient reply!

I just saw your note. Somehow, it got lost in archives. You are very welcome. :)