It was the developers who decided to do this. If you are finding it impossible, there is a fork of Thunderbird called Betterbird that does not have the threading default. It isn't supported here, but it's virtually the same product. You may find happiness there.

Tell the developers to make an option for NO THREADING. All-threading is a deal-breaker for me - it makes it impossible to locate messages by date!

See https://support.mozilla.org/en-US/kb/message-threading-thunderbird

I agree with your thoughts, but that won't happen. The following steps do work and have been confirmed by users: 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 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  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.

This isn't a perfect solution, but it does address the issue of unthreading many folders.

Many thanks for the proposed solutions. Unfortunately, there seems to be no universal solution for PERMANENTLY turning ALL threads off without having to apply the unthreaded sorting to each of the parent folders (simply choosing account name or Local Folders does not seem to work) and even then it is not certain that all child folders are affected. And much as I appreciate the option of changing settings (including the default threading mode) through the Configuration Editor, I am not sure this is quite user-friendly as far as such basic settings are concerned.

Yes, it is not user-friendly, but that is how the developers set it up and I doubt this changes, at least not for the foreseeable future. There has been some disagreement and displeasure in that decision, but we, on the support side, have no voice. The config editor is not complex and is a common tool for adjusting settings and I encourage you to do that to allow you to create new folders that are unthreaded. Reports from those who applied the proposed steps as I outlined above indicate that the process works. I regret there are no other options.