Review for XNote by Martijn
Rated 5 out of 5 stars
It's a great extension for keeping notes associated with specific messages -- especially the user interface is very convivial: post-it notes which can be placed anywhere on the screen (and which reappear anytime that same message is selected).
Only thing I'd like is that it would show somehow when a note has got the focus.
To create your own collections, you must have a Mozilla Add-ons account.