Most of the setup is similar to the Watchguard Firebox II except some hardware differences. The PCI slot is on its side, you will have to raise the board out of the case to perform this, if your video card is too large. I used the same video card as the Firebox II, an S3.
Also the keyboard header has changed, after some playing around, here is the pinout!
WG Keyboard Pin 1: PS/2 Header Data
WG Keyboard Pin 2: PS/2 Ground
WG Keyboard Pin 3: PS/2 Header +5v (VCC)
WG Keyboard Pin 4: PS/2 Clock Pin
The rest worked like the previous Watchguard, enjoy.