Rated 4 out of 5 stars

It works!

Looking forward to a more polished UI. In particular, I'd like it to be visible at all times, and to highlight the "From" email's domain as either green (verified), orange (unsigned) or red (verified-wrong).

This review is for a previous version of the add-on (0.3.0). 

Your suggestions are added in version 0.4.0. If you don't mind installing a unreviewed version, you can download it already from https://addons.mozilla.org/addon/dkim-verifier/versions/