In support of previous feature request Rated 5 out of 5 stars
I think that this https://addons.mozilla.org/en-US/firefox/reviews/display/10704#review-165462 feature request makes sense, do you know probably of any other tool which can do that?
Btw. since the addon is called "CSS Usage" I sort of even expected it to do what @xybermatthew describes, i.e. being able to extract CSS Usage inside the particular part of the page