Great Idea / Actualization ok Rated 3 out of 5 stars
The idea for this add–on is really great and I've been looking for exactly this kind of tool for quite a while already.
Unfortunately the arrow buttons to "nudge" the overlay by pixel don't really seem to work for me (FF3 o OS X) very well. I can usually click once or twice, after that I'll have to hover off the button and buck on to be able to click again.
This observation is not valid for the "opacity" value.
Besides that the whole menu shows up twice in my FB.
Keep up the good work!