Once you change the default setting, it applies to all folders created afterwards. For existing folders, you can reset them to unthreaded. Steps for this and more are here: https://support.mozilla.org/en-US/kb/message-threading-thunderbird

Just so you understand, the setting is at the folder level, so changes must be applied to each.

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.

Many of us would LOVE to see the default changed to unthreaded, but that seems unlikely.

Thanks, David, but I have hundreds of Local Folders and sub-folders created over twenty years, so your first solution is impractical. I assume that the only solution is thoroughly uninstalling Thunderbird and reinstalling it from scratch using the method you have indicated. That would be a major headache, though, in view of all the custom settings that would need to be reset and of all the add-ons that would need to be reinstalled. Is the Thunderbird development team that rigid? If most of us users would love to see the default changed to Unthreaded then why won’t they do it? Same goes for the placement of the menu bar.

There is a simpler approach.

These steps may help: To set many *existing* folders, follow these steps: This must be done on an account by account basis.

1. select table view - the icon just to right of 'quick filter' button above message list pane
2. select a message folder and apply all desired attributes - View>sort by from menu bar (if not shown, click alt key)
3. click the icon at right side of column display in message list pane
4. select the entry near bottom 'Apply current view to...'
5. highlight 'folder and children'
6. 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

If the above seems complex, here are the steps again: - set one folder to your preferences and highlight it - click the icon on right side of the menu bar of message list pane - click to apply current view to> and highlight 'folder and subfolders and PAUSE - a menu opens to left with account name. highlight that and PAUSE - another menu opens to its left with account name at top. CLICK THERE and a confirmation message will appear to confirm setting all folders in the account. Repeat for all accounts.