Full instructions are here: https://support.mozilla.org/en-US/kb/message-threading-thunderbird

Here is my own documentation on threading, duplicating the above, but it may help: 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  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.

You lose me on the last two steps:

Hover over an account (please define "an account" if you would so that I know where to hover.

Where does this "next menu" come from?

 -> In the next menu don't select a folder. Instead select the account name itself.

Thanks for your assistance with this.

By the way, I am trying to get my INBOX unthreaded. Is this considered a "folder"?

Yes, the inbox is a folder. By 'next menu', I am referring to the small menu that pops up when you drag the mouse. Please revisit my crude video. It highlights my Gmail inbox and then sets the Gmail account. https://davidkirk.org/SettingDefaultFolderView.mp4

I unthreaded a folder, then went to apply current view t and then to my account and asked to unthreaded it and its children.

No effect....

pls help, this threating is a PITA. You cannot erase a single note without the risk to loose aal.

I fully agree on the PITA aspect. The unthreading steps are clumsy and may take several attempts to succeed.

As far as I am concerned, it is "not to succeed".

Mozzilla did a major effort with the new version of Thunderbird, but the result is still below standard.