External scans are more of looking at your home network from outside. And ports discovered here can be
1. Ports Open Using UPnP
If you have these, you can find the port forwarding in "Ports opened by UPnP", you can tap on it to block it.
2. Ports Open Using Static Port Forwarding
These are likely configured by you on the router. Such as port forward to one of your local devices. Firewalla can not manage this, so you will have to delete the port forwarding entry in the router.
3. Ports Open by the router
Very rarely you will have a router that opens ports to the outside. This usually is a configuration problem, please check your router's manual and close it.