Earlier quoted context omitted.
>we don’t know what the user wants How about they ask their users on first install?
So everyone clicks "no" (or gets confused and asks a technical relative), Firefox always sends the DNT header, so no sites respect it.
Isn't that a good thing for users to know about tracking? Firefox can always include some small popup help with the prompt explaining the DNT.
>so no sites respect it
But they will lose a big excuse(it's default) not to support it.