Doesn't work with firefox 3.0 - seems like you'd be better using two elements, transparency, and z-index. This "old fashioned" way the shadow will work on all browsers it doesn't take much more code than his example.
I don't think that gives you a way to achieve a feathered shadows, does it?
Plus, adding another element just for the shadow is unsemantic - if that's something that matters to you.
Firefox 3.0 has now reached EOL for security fixes. If you’re still running it you’ve got much bigger problems than missing drop shadows.