This add-on is, always has been and always will be FREE. The suggested contribution is for DONATIONS. It is also bad form to write a review about a compatibility issue (being that it hasn't been updated for new versions of Thunderbird), because the add-on is not advertised to work with anything past 3.1.x.
Every part of your review reeks of poor form. Nothing is even remotely what a review is supposed to be about.
This add-on delivers EXACTLY what it promises and therefore it gets 5 stars from me, as I have read the review guidelines.
This review is for a previous version of the add-on (2.1.2).