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.

The setting in config editor applies only to folders you define after the change; there is no change to existing folders. My guess is that you have not gone through the multiple-step solution to set all folders as unthreaded, which would indicate you have some folders threaded and some unthreaded. Here are the basics: <pre> 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>