------------

   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.

ALSO, for future installations of Thunderbird, 
- install Thunderbird and exit prior to setting up accounts
- create a simple text file with name of user.js with these lines: 
     // Set default threading to UNthreaded  ###   <== this line is optional, just for documentation
     user_pref("mailnews.default_view_flags", 0);
- copy the user.js file into the profile
- now, when you start Thunderbird, unthreading will be the default. Problem solved. 

The full set of view options are in the menu bar, which can be viewed by clicking the Alt key. The instructions do work, and have been confirmed by many users. The steps I documented show selecting one unthreaded folder and then applying the change in a few key clicks to all folders within that account. And to repeat the process for other accounts. The other approach is to set the config option prior to setting up accounts. Here is a repeat of my instructions: 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: 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: <pre> 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. ALSO, for future installations of Thunderbird, - install Thunderbird and exit prior to setting up accounts - create a simple text file with name of user.js with these lines: // Set default threading to UNthreaded ### <== this line is optional, just for documentation user_pref("mailnews.default_view_flags", 0); - copy the user.js file into the profile - now, when you start Thunderbird, unthreading will be the default. Problem solved. </pre>