The Firebox II's are Pentium 200's or AMD K6-300's, and will run m0n0 well 
if you can get a working image on the machine. :)

I have an older post on the subject (from Jan/Feb '05), but in a nutshell, 
the FBII has an internal 44-pin laptop IDE connector and a jumper that can 
be used to switch between booting the 8 MB built-in flash memory and the IDE 
port (which will only boot HDDs, not CDROMs). Although it isn't readily 
apparent, you can plug in a video card (into one of the 2 on-board PCI 
slots; they are BACKWARDS and the "exterior" side of the card should face 
the front of the box) and a PS/2 keyboard which lets you get into the 
stripped-down BIOS; this is mostly useful for drive geometries.

The m0n0 version that you install MUST be a custom kernel with the psm0 
(ps/2 mouse) detection/support removed; this probe hard-locks the box on 
boot.

Basically, you end up with a sequence of: build a custom IMG with a custom 
kernel, set up a boot drive, boot the hard-drive and copy the custom image, 
flip the jumper and you're off. Once a working m0n0wall image is in the 
flash, you can use the console serial port for initial setup.

I had moved from a FBII to a Nokia IP110 but just recently have been working 
on getting my Firebox running 1.2b9 but have been sidetracked.

Joe