Rated 5 out of 5 stars

Honestly, when I saw the "needs access to everything on your computer" permission I was worried so I downloaded the code first and had a look through it. The code is really minimal, there's not much to it and the only thing it does is what it says.

This review is for a previous version of the add-on (1.0.1). 

Thank you for confirming it is safe!
Further information:
https://support.mozilla.org/en-US/kb/permission-request-messages-thunderbird-extensions#w_have-full-unrestricted-access-to-thunderbird-and-your-computer