Rated 4 out of 5 stars

It works in FF4.0! "Right-click on a toolbar then choose "Customize". Look for the ruler icon in the popup that appears and drag it where ever you'd like it to appear."every_man is right - This needs to be on the plugin page or the icon needs to placed somewhere by default.

EDIT: Stopped working today, maybe interference with some other addons.

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