It is possible for ISPs / webmail service providers and distributors of mozilla based applications to extend the message pane. For example, to change the color or display special icons.
For now, this is just a place holder document.
For starters, see: http://bugzilla.mozilla.org/show_bug.cgi?id=161226
One day, we'll provide some samples to use it and more documention.
If you have questions, send us mail.