I've seen zero animated favicons. I'm wondering if it's a "feature" that doesn't work on Linux. The URL bar doesn't even display favicons anymore, although they show up without animation in bookmarks.
http://www.p01.org/releases/DEFENDER_of_the_favicon that somebody linked to shows that the animation does indeed work in the Linux version of Firefox. Weird that I've never seen one before, but now that I've seen it, I don't want to see another.