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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: '''FOR NEW FOLDERS''' 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. ------------ '''FOR EXISTING FOLDERS''' These steps may help: 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. SETTING UNTHREADING: STEP BY STEP ############# Here is official info: https://support.mozilla.org/en-US/kb/message-threading-thunderbird The link also explains how to use config editor so that all future folders you create are unthreaded. On your question: - set one folder to your preferences and highlight it - click the icon on right side of the menu bar of message list pane as shown on web page - 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.

Thanks for the detailed answer.