Freaking Awesome!! Rated 5 out of 5 stars
With lot's and lot's of projects at once this add-on is a musthave for everyone. I put the current project folders that I work on, on top to easily manage every e-mail coming in and out. This add-on should be a default install on every Thunderbird version.This review is for a previous version of the add-on (3.5).
...and not to forget:
also install the new companion extension, quickFilters, for even more added email sorting powers!