IE has lots of issues with favicons. It often caches them for a long time (meaning if I change the one on this site right now, many of you may not see the change for weeks or months. The other issue is that IE is much more finicky about the actual format of the .ico file. FIrefox will allow .png and even gifs or bmp files with 16 colors renamed to .ico .... IE will balk at many of them and show nothing.