This is and isn't an issue. By default, intranet authentication (spnego) is disabled in Firefox. There are numerous guides explaining how to turn it on, and it

This is and isn't an issue. By default, intranet authentication (spnego) is disabled in Firefox. There are numerous guides explaining how to turn it on, and it takes all of a couple of minutes to set up correctly after which point it just works. FWIW IE has this problem too, in that by default it only enables negotiation for unqualified hostnames. If you want people to use an FQDN, you have to go add that FQDN to a list buried behind a couple of advanced dialogs.