Rated 5 out of 5 stars

I downloaded B6 but I am unfamiliar with the file type? How do I get that to work in Thunderbird?

This review is for a previous version of the add-on (12.0.4).  This user has a previous review of this add-on.

NA
The file is an XPI which is just a zip container for the addon. To install go to Tool/Addons and then to the gear button. Choose Install Add-on from file and locate b6.

If you have issues please raise an issue in Github so we can look and interactively solve the problem.
(also reconsider review for a beta question...%-))
Cheers
Christopher