In the Message List view, in the column header on the far right, open the options menu.
  At the very bottom is the option:
  -> "Apply current view to..."
  -> "Folder and its children..."
  -> Hover over an account
  -> In the next menu don't select a folder. Instead select the account name itself.

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: In the Message List view, in the column header on the far right, open the options menu. At the very bottom is the option: -> "Apply current view to..." -> "Folder and its children..." -> Hover over an account -> In the next menu don't select a folder. Instead select the account name itself. This isn't a perfect solution, but it does address the issue of unthreading many folders.