If you could find a couple of extensions on the boxes outside the LAN (calendar, the french pack, and checky on the Linux box), the server itself hosts a brand new FF, installed specialy for the disagnostic. No extension there.
I'll try to install FF on the Windows server which is on the LAN, but I would be very suprised not to experience the same poblem.
1) tested with Mozilla 1.7.8 on the server : same issue.
2) tested with Epiphany (another Gecko) on the server : same problem.
3) tested with Konkeror on a Linux box outside: idem.
4) tested on IE6 on the same Linux box as used for Konkeror (via Wine and the ies4linux installer): it works!
1 & 2 => the issue is not with FF but may be with Gecko
(1,2) & 3 => not only Gecko, but khtml also
preceeding tests & (1,2,3) => not a LAN/WAN issue, neither Windows/Linux
4 => it's definitely a non-IE issue.