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:
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.

I did a simple (okay, crude) video at https://davidkirk.org/SettingDefaultFolderView.mp4


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. 

You're right, that looks like an error in instructions and I'll report it. The instruction works if you use the menu bar, not the hamburger menu. To follow its intent, if menu bar is not visible, click alt key and then click view>sort by>unthreaded. Here are more complete instructions if you have many folders: 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: 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. I did a simple (okay, crude) video at https://davidkirk.org/SettingDefaultFolderView.mp4 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. </pre>