great idea but Rated 5 out of 5 stars

I would prefer it use another visual indication rather than injecting code into the current page. consider instead

+ changing the graphic within the location bar (as is possible now in CSS)
+ using slide down notifications (as when attempting to install an extension from a non-whitelisted location)
+ the same slide up notification area NoScript uses (above the status bar) and preferably color it something else

This review is for a previous version of the add-on (0.8).