After upgrading to Firefox 1.5, I noticed an animated favicon in a tab (the favicon being the small image shown in the tab to the left of the title, and also next to bookmarks). Other people report seeing the same thing, also after upgrading to 1.5. Draw your own conclusions.
UPDATE: A reader reports that he also sees the animations on Firefox 1.0.7/Linux. Is this a relatively new feature, or have designers just discovered it, or am I just noticing an old feature?