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.

Thank you for the reply. Unfortunately, selecting several folders had no effect (thus why I asked the question here).

I will try the proposed solution. I wonder why did Thunderbird thought it would be a good idea to make this so complicated?

Many of us share your question. Fortunately, the solution is rather straight-forward.

I have the latest version, tried everything here, no hope. I tried You need to highlight each folder and click View>sort by>unthreaded. BUT when I highlight a folder I cant find view etc. anywhere.

Please reread the instructions and view the video. You need to repeat the process once for each account, not each folder. If you are not seeing VIEW on menu, press Alt key and menu bar will appear. Then press View>toolbars and tick Menu Bar. There is also a link to the View option in upper right of main screen.