Question about Direct Connections

Hey all, I have a question about direct connections. Limewire (4.8.1) says I'm behind a firewall and cannot recieve direct connections, but I gave my ip to my friend fine, and he can browse and get stuff from me, and I from him (Same version no.). This confuses me. Also, we have very similar internet setups, (ie, same router, same service provider, same connection speed, etc.) except he's on a windoze and I'm on a Mac. He gets 4 stars, and he says I have a measly 3. What gives? It's not a huge deal, but I'm just wondering why Limewire doesn't recognize the fact I've got my router set up to allow direct connections. Thanks for the help.