For preexisting folders, whether input or sent, you must manually reset after changing the default.

Unfortunately, as described in the initial question, manually setting "View" -> "Threads" -> "Ignore threads" only works in "Inbox" and is ignored when set for other folders. The problem is that the functionality to disable threads does not work outside of the inbox. Thanks in advance. John N.

If you are wanting to disable threads, you need to use View>sort by and click unthreaded. And the documented instructions are clear in that, if you set inbox (or sent folder) to unthreaded and then click to apply to all folders, it also changes the sent folder. What you are saying is not true.

Thanks for replying. Where is the "Apply to All Folders" menu option? Mozilla's documentation states it should be under the "message list display options" menu but that selection is not present in Thunderbird 115.12.2. The "message list display options" menu now only includes: -Table View -Cards View -Sort By (It's not a submenu option here either) -Hide Message List Header

Thanks in advance, John N.

in table view, the icon on right side of message list pane, apply current view to... This is well documented on website and here. A search on this forum may surface it....

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:
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.