Firewall supports all the hardware that is supported by a standard NetBSD install.
That's quite a lot, and the list is updated all the time. For a recent list, please check out the NetBSD web site. A minimum install of NetBSD is 4 MB of RAM on a 80386, but for a busy firewall, we recommend a bit higher than that. Any i386 or better CPU should work - genuine Intel or a compatible such as Cyrix, AMD, or NexGen. See the XFree86 FAQ for more information.