The Cobalt Firmware is only used on Cobalt MIPS systems. It consists of a Linux 2.0 kernels which can read a gziped ELF kernel directly from a serial line at 115200bps, 8N1. When booting from disk the firmware kernels expects the kernel binary to be booted to reside on an ext2 filesystem /dev/hda1 as /vm.gz.
Firmware can't mount the boot partition
The firmware is based on a Linux 2.0 kernel. This kernel only supports ext2 rev. 0 filesystems. Current e2fsprogs however default to rev. 1. Journaling isn't supported with rev. 0 filesystems either so the partition containing the kernel cannot be ext3.
Kernel is too large
Due to unfortunate use of the available memory the size of the bootable kernel is severly restricted. In Linux 2.6 even a minimal kernel will exceed this limit.