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.

David, I found your response and I have done this before. Unfortunately, when Mozilla pushes out a Thunderbird update, it seems to reset everything. Also, I have not found ANYTHING that will fix the message thread view for existing emails (hundreds of them). I reapplied the mailnews.default_view_flags 0=unthreaded change and did the set all folders/subfolders like this one change. It would be very nice indeed if someone would PLEASE take mercy on us and give us an option to TURN OFF message threads with one click, including reverting the view for any existing emails. Thanks.

That complaint is posted almost daily, but it does not appear likely to change, as it was introduced in the summer of 2022. However, once set, it should remain, as the setting is in each individual msf file for folders, not just one place.