You can't directly scan the external side of your firewall/router from your PC if your PC is connected to the internal side. With a tool like Ethereal you can only see/capture the packets at the point at which you are connected.
However If I understand correctly what you're asking, then there might be another way to achieve what you want. If your firewall/router has any kind of logging that you can turn on, you could get it to log the traffic that it drops (prevents from getting through to your protected PC). Let us know the type of router and we'll see if anyone can advise
With that enabled with the IP address of your machine it will be like you are directly connected to the Internet however you get all the security risks that come with it too but you should be able to see some Internet traffic this way, It's just a bit unsafe compared to Bishops suggestion. Also if you do it this way be sure to have a firewall application installed on your machine like