Well, solved my own problem.
A little work with Ethereal revealed that the Rokus were getting back the wrong IP address from their initial queries. It turns out that the machine running Firefly was multi-homed (2 IP addresses), and was returning the 2nd IP address to the Rokus, which were unable to access it.
As it turned out, I no longer had a need for the multihoming, so I turned it off and all is now working – excellent!
Jeff