That site doesn't prevent caching of their icon but Firefox isn't saving the icon. I suspect it's because Firefox first requests https://login.paysafecard.com/favicon.ico and that redirects to a login page instead of returning a "not found" or "forbidden" response. Firefox does proceed to request the real icon at https://login.paysafecard.com/customer-auth/clients/mypins/favicon.ico?v=2.78.1 and uses that on the tab, but for some reason it doesn't use that one for the bookmark. ??