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. 
  ------------
	
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 is the official (to my knowledge) documentation. Well worth the read 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: <pre> 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. ------------ 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. </pre>