If your favicon displays in Firefox but not Internet Explorer then delete your browsing history in Internet explorer.
I came across many threads on this topic but basically if it is showing in Firefox then you have a good favicon image and it is in the right place in your website. Apparently IE has a mind of its own as to when it refreshes the favicon for a stored page and from what I can gather it is every 2 weeks or so. The easiest way around this is to delete your browsing history.
