Rated 4 out of 5 stars

Fantastic little application, for leaving useful notes about specific messages. Great for helping to organise work and tasks.
Works fine with TB 2.0.0.17 (Windows XP) also with Thunderbird installed as a U3 application. Xnote displays a re-positionable yellow post-it type note when the email associated with the note is at any point selected. Notes can be deleted (obvious cross bottom left) or hidden (cross top right).
Creates a new column in the email pane and emails with xnotes attached can be seen. Adding an Xnote to a specific email can be done either from right click menu or customise TB's toolbar to include the add Xnote icon. Easy to use, just be prepared to explore TB to customise its functionality to suit you.

Only down side is that Xnotes are associated with the local machine being used to create them. So if you read emails from several locations. The note/s created on one machine are not viewable on another... Now that would be VERY useful!