Yes, here is the fix: In config editor, you can change default that applies to NEW folders you set up: mailnews.default_view_flags 0=unthreaded

Config editor is at tools>settings>general and scroll to bottom right.

Thanks David, you seem to be onto something but we are not there yet. When I search for mailnews.default_view_flags in config editor the system tells me that its value is 1, but I cannot edit it. Please assist.

Look at right side. There is an 'edit' button. After editing, save and then bring up config editor again to be sure you saved it.

Thanks for that. I edited in config editor as per attached, then closed that tab, then I closed and restarted Thunderbird, created a new subfolder, then copied e-mails into it. As you can see they are still sorting by threaded. Returning to config editor I see that the edit of mailnews.default_view_flags has not worked. -Can you explain why? How do I force it to accept an edit?

After changing, press enter key a couple of times and then save. Immediately reopen the editor to confirm that it changed. No need to close and restart.

My question is why is this buried and why did it change. Your short hand description of where to find this setting is not enough. Okay I am finding it now I think.

I want to offer some constructive criticism, tell us where to look, then what to look for. If someone goes into the tools settings window all the time what you wrote is very easy to follow, for someone that does not it is not easy to follow. The fist two steps are on the menu, then selecting from the left side of the screen, then the body of the window. You directions look all the same, but they are all over the page. I spent over 30 years writing instructions for users to follow and I learned if I tell them where then what they are much easier to follow.

I got into the Advanced Preferences and have no clue what to do now. I am not seeing anything in it at all. I believe mine is blank right now, do I add this as a boolean? mailnews.default_view_flags 0=unthreaded

Config editor is at tools>settings>general and scroll to bottom right.

This is easier to follow.

From the menu select Tools, then settings. On the left side select General, Scroll to the bottom right and look for the Config Editor button.

This is only one of many annoyances that have come up as Thuderbird updated itself. From was replaced with Correspondents in all folder, what the heck! ARGGGGG

After View/Sort by/Unthreaded then propagating that by "apply current view to all folders." All folders do in fact sort by unthreaded.

I got the View/Sort by/Unthreaded, but where is the "apply current view to all folders" ?

For the 'apply current view....' look at menu bar atop the message list pane, right side. Click there and a drop-down menu appears and at very bottom is the 'apply current view...' menu.

I have followed your instructions. No matter what I do Advanced Preferences / mailnews.default_view_flags insists on reverting to 1. Please assist.

- click edit, change value to 0, icon on right changes to show checkmark, click the checkmark and then exit.