If you flashed a WRT54G to DD-WRT then you gained quite a bit! I run several version 3 54g routers and the first thing I do is flash DD on it.

This page will show you which routers are supported.

https://www.dd-wrt.com/wiki/index.php/Supported_Devices

Firebox is not supported. Why would you want to mess with Watchguard's OS anyways? They make fairly robust routers.