Highly Recommended for Web Devs Rated 5 out of 5 stars

Very nice. Thanks to Orev for clearing up why I wasn't seeing the add-on. I completely glossed over the fact the add-on bar was disabled.

This review is for a previous version of the add-on (1.4.1-signed.1-signed). 

The default Firefox settings now hide the Addon Bar. You need to right-click in the empty space next to the tabs and select "Add-on Bar" to enable it. If you don't like the whole bar across the bottom, you can try the "Barlesque" extension.