Apple:
If you own an iMac (Slot Loading) computer, you must install the iMac Firmware Update 4.1.9 before installing Mac OS X 10.2 or later.
The iMac Firmware Update 4.1.9 is available on the Mac OS X Install CD Disc 1. It also available online and can be downloaded if needed.
To install the iMac Firmware Update 4.1.9, your iMac (Slot Loading) computer must be started up from Mac OS 9.1, or 9.2, which must be installed on the hard disk. You cannot install the firmware update if the computer is started up from a CD-ROM, or a network disk. If you normally start up the computer with Mac OS X, you must start up from Mac OS 9.1 or 9.2.
I remember clearly walking through these steps with the handful of slot-loading iMac G3s we had at the high school newspaper.