Jason: Unless you intend separating a portion of your internal network from the rest of it then it would appear you are scanning from the inside with those IP addresses. The following address blocks are reserved for private networks and routers on the net will not route packets destined for them.

192.168.xxx.xxx
10.xxx.xxx.xxx
172.16.xxx.xxx through 72.32.xxx.xxx (IIRC)

You need to be at a location remote to your network and know the external address of the firewall to be able to properly scan it.

[Edit]

Angelic: A closed port can't be "exploited" in the traditional sense. It can be used to determine OS type but actual exploits can't work because the packets received on the closed ports are not acted upon. The proper thing for the closed port to do is to simply respond with an RST or RST/ACK.

Keyser: Because the packet is responded to with an RST or RST/ACK the scan tool knows the port's state and can move on. When the packets are dropped the scanner must try several times in case the packets was lost in transit. Each time it tries it must also wait for an allotted period of time before it retries.... Hence, scanning a firewalled machine usually takes quite a bit longer then an unfirewalled machine.

[/Edit]