At the moment Monowall doesn't support any modems USB or otherwise. I don't think that it is part of the brief that Monowall is designed to. On the
Mono homepage it states.
"m0n0wall is a project aimed at creating a complete, embedded firewall software package that, when used together with an embedded PC, provides all the important features of commercial firewall boxes (including ease of use) at a fraction of the price (free software)."
Support for modems is not normally found in commercial Firewall routers. As far as I know it is not part of the plan for Monowall. There is a project called
Smoothwall that does support some modems, although the feature set in the Express (Free) version is not nearly as comprehensive as Monowall, but if you are just looking for a simple home built firewall, have a look at it.