It rarely works, Most of the time the top option is "-apple-system" and the rest are CSS style values.

Works well with most sites, although not all of them, such as addons.mozilla.org, where it shows the font details of the site you were on previously. Otherwise a really handy add-on.

(Would also be nice if the context menu showed size in points as well as pixels, separated line height from size, and prefixed each line with what it means, e.g. family, weight, size/line height, color.)