// Set default threading to UNthreaded  ###   <== this line is optional, just for documentation
   user_pref("mailnews.default_view_flags", 0); 

This is the official (to my knowledge) documentation. Well worth the read https://support.mozilla.org/en-US/kb/message-threading-thunderbird Here in more detail: '''FOR NEW FOLDERS''' 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. '''FOR CURRENT FOLDERS''' These steps may help: To set many *existing* folders, follow these steps: This must be done on an account by account basis. # select table view - the icon just to right of 'quick filter' button above message list pane # select a message folder and apply all desired attributes - View>sort by from menu bar (if not shown, click alt key) # click the icon at right side of column display in message list pane # select the entry near bottom 'Apply current view to...' # highlight 'folder and children' # move mouse left to see account name and move left again to display all account folders. Now CLICK. a small menu will appear to confirm setting all folders within that account. '''FOR FUTURE SETUPS''' ALSO, for future installations of Thunderbird, - 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); '' # install Thunderbird and exit prior to setting up accounts # copy the user.js file into the profile # now, when you start Thunderbird, unthreading will be the default. Problem solved.