I am using the following:
<link rel="shortcut icon" href="/images/favicon.ico" />
It is a true 'ico'. When I visit http://mydomain.com, the icon loads. But when I visit the 'www' subdomain: www.mydomain.com...it won't load. Any ideas what is going on?
I found that I had to clear my Firefox cache [CTRL]+[SHIFT]+[DEL], and then restart Firefox before I could see the favicon, which I put in the root of the web server and called favicon.ico.
Note that in recent versions of Firefox the favicon is only displayed on the tab icon and bookmark, not in the address bar icon.