Rated 5 out of 5 stars
*Update - the add-on works fine. My problem occured because of incompatability with Thunderbird Conversations add-on. Just have to decide which one I'd rather keep now :(
If this worked properly on Tbird 14 I would give it 5 stars. Being able to hide the folder pane and thread list with one key would be very convenient.
The problem is toggling message maximum works, but toggling back to the normal message size doesn't. It shrinks the message until only the scroll bar is left. So you'll have your folder pane, thread list, then a blank frame where the message would normally be.