Older firmware updates do not run in Mac OS X, they only run in Mac OS 9.
If the firmware you need to upgrade to is only available as a Mac OS 9 firmware upgrade, it is necessary to either install Mac OS 9 on the machine first, or to boot from an external drive running Mac OS 9.
Furthremore, what makes this extra complicated is that firmware upgrades must be run from a writable boot partition, which means you cannot run the firmware upgrade if you boot the computer in Mac OS 9 from a CD. It must be done from either the internal or an external hard drive (which is writable).