The only sure way to avoid threading is to set the default to unthreaded when first starting Thunderbird.

- install Thunderbird and exit prior to setting up accounts
- 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);
- copy the user.js file into the profile
- now, when you start Thunderbird, unthreading will be the default. Problem solved. 

If you have already set up accounts, life is harder.

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.

(NOTE: My earlier suggestion for user.js does this for you, so that step isn't needed if you do the user.js)

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To set *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.

Thanks so much for this help. I can change for all folders, but NOT FOR the search results, which always show as threaded - the most annoying! Let's hope for the next update.... David

You're welcome. Several users have commented on the threaded search results and I have idea of the fix.