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 at

   https://davidkirk.org/SettingDefaultFolderView.mp4

  In the Message List view, in the column header, on the far right, open the options button that shows    
   context menu. (See attached screenshot):
  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.

102.6.1 (64-bit)

David,

I followed those steps (your reply - above; your video; https://support.mozilla.org/en-US/kb/message-threading-thunderbird) but the view was not applied to the other folders.

In my case I had removed the threading/grouping (View > Sort by > Unthreaded) and then followed the steps to apply to all other folders.

Upon moving to other folders they were still showing messages grouped/threaded.

I ended up going through and manually applying (View > Sort by > Unthreaded) to all folders.

It may be difficult to see, as it is the bottom option on the menu in video that does it. The one above just sets column view.