*Correction* -- Workaround for Mac, FFox 27.0.1. Rated 4 out of 5 stars

Er, correction to my post here:
https://addons.mozilla.org/en-us/firefox/addon/pixel-perfect/reviews/556100/

... Path should have "Library" in it:
Users/USER/Library/Application Support/Firefox/Profiles/SOMECODE.default/extensions/pixelperfectplugin@openhouseconcepts.com

For the users whom are not so techie I noticed in these reviews, keep in mind Library is a hidden folder by default, so you'll need to reveal hidden folders in your Mac's settings.

This review is for a previous version of the add-on (1.8.0.1-signed).  This user has a previous review of this add-on.