Rated 1 out of 5 stars

DO NOT INSTALL! Previous "Deselect on Delete" did not require any permissions. This version wants "full, unrestricted access to Thunderbird and your computer."

There is no reason for this add-on to require such broad permissions. Seems like a backdoor.

This review is for a previous version of the add-on (3.1). 

It's not a backdoor. You can unzip the extension and look at the code, it's small and clear. Requiring this permission is needed to use "experiments" API. Previous addon required same permission. Btw when installed, both will show "This extension doesn’t require any permissions", it's mozilla bug.