How to turn off threading permanently

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

The last version without threading was 91 The setings>appearance feature is available in 140. Updating to 140 is the cleanest approach, going forward. You can find your prior version in settings>general> in the Updates section by clicking the 'show update history' button. To stay where you are and set default for unthreading for NEW folders, do this: '''FOR FUTURE THUNDERBIRD INSTALLATIONS''' # -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 (NOTE: Config editor is at settings>general near bottom of pane) # - 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. # That sets default for new folders. Existing folders must be set manually as you indicated.