grml boots via firewire. Thanks to Martin Buchleitner for helping me to locate the problem, now we will officially support booting grml via firewire starting with the upcoming release(s).
BTW: I renamed the 'usb'-cheatcode to scandelay and slightly improved it. Now it's possible to add a delay of some seconds to the boot sequence so the modules in the initrd have enough time to access the devices. Currently scandelay uses a default of 10 seconds, using a parameter makes it more flexible - e.g. scandelay=3 will add a delay of only 3 seconds.