Tried the extension and it suits my needs as a glove. I did notice something frustrating tho.
After installing the plugin/extension I saw the message note plus on all mt emails, always visible at the bottom of the message window regardless if there was a note or not. After checking the extensions preferences it disappeared for emails that did not have any notes.
Anyhow, this is a bug and a feature request.
The bug: Message notes plus shows up on all emails whenever it should or not.
Feature: a option to enable message notes plus to always be visible on all emails.
Using Linux mint 13, Thunderbird 15.0 and message notes plus 18.104.22.168