I mean, I downloaded the code for the MB8600 from Motorola Cable's GPL compliance center and it's literally just a Linux appliance. You can set firewall rules any way you want on any network interface the system sees.
If Cox's configuration file includes a rule that blocks traffic originating from localhost:8080 (in this case, the modem) from going over the Ethernet interface, they certainly could block this specific page.
Alls I knows is before I plug into Cox's network, the page loads but doesn't do anything because the page needs to have a CoAX cable plugged in. When I plug into Cox's network, it stops working.