Looking at the source for Skype.com, I cannot find any <link rel="shortcut icon"> tag (or anything similar), and yet, the icon is clearly showing up in FireFox. Where are they defining the favicon?
Unless otherwise defined, the browser will look for the favicon in the root with the filename favicon.ico
As far as I know you need the exact name favicon.ico for the browser to detect it. That might be different in other browsers, but if you want to use .png instead of .ico for some reason it would be safer to call it out manually for browser compatibility even if the newer browsers are seeing it.