Not quite there yet for me Rated 3 out of 5 stars
I would love to give this add-on top marks, but unfortunately whenever I install it my Firefox gets really unstable and prone to freezing. It's obviously a conflict with something else I have but I positive it's there.
It's too bad because I really would like to have an inline css editor that works reliably. One with line numbers and search. Even Firebug falls way short of this goal. But I have to commend the developer for the effort and I will keep checking if maybe one day that conflict disappears.
(FYI, I use several "portable" firefox instances that allow me to test FF in a variety of configurations. The common denominator in teh unstable ones is always EditCSS paired with the usual web development suspects like web dev, firebug, etc.)