I have the same problem. Tried the solution suggested by ProtoSpec, and managed to find that setting in my router, but no dice the problem persists.
I have tried the firebind test from another computer in my house, and got the same results, so it's likely a router problem.
Router is a D-Link DIR-615, PC is a Windows 7, with avast, Windows Firewall and Microsoft Security Essentials running in background.
Ports are forwarded according to this article.
EDIT:well that's odd. 2 unrelated reboots later, and it starts working fine. I'm just going to think of this as a gift horse and let it go.