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!