Don't know much about network protocols etc. Does firefly advertise my firefly music server over my ADSL router to the outside world? Or does it automatically limit itself to my local 192.x.x.x home network only? If not, how do I limit it?
No, it shouldn't. Your router should block those. Some routers have an option to specifically enable it, but I haven't seen any that would automatically enable it.
If it did get past the router, your provider is probably filtering that (as well as martian addresses, etc) on their end, as well.