------------

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

I concur. Complaints on threading have been active here since it was first rolled out with ver 102. They shot themselves in the foot with that and I doubt it's ever changed back. SO, we have to fix it ourselves. Here is a full set of instructions: 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. ------------ The above is ONLY for NEW folders you create in the future. 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>