First, it's not easy. 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 button that shows context 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.

Thanks. I've seen this thread but when i go to Tools/settings/general , i don't know what to do anymore.

The config editor is at lower right. Click and type in

    mailnews.default_view_flags     and change value to 0    That sets any folder created in future to unthreaded. It does NOT change existing folders.

I think "mailnews.default_view_flags" was missing in your above message. But still didn't work.

I can't see this: -> "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.

Sorry mailnews.default_view_flags was in your above message

HI, in

 In the Message List view, in the column header on the far right, open the options button that shows context 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.

I found it thank you. It works a treat