Thunderbird has been doing that since 102 was released. This URL has the full info on changing on a folder by folder approach, or set default for NEW folders. https://support.mozilla.org/en-US/kb/message-threading-thunderbird

Thanks

Thats a pain, i have about 150 folders to go through and change 1 by 1 then

Why do they insist on changing things? This is why i seldom do updates, just messes up what i have.

Use this 'quickie' approach. Yes, I agree they really shot themselves in the foot on this.

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.