Firmware is the software that controls the basic functions of the iMac's hardware. Firmware is typically stored on a ROM chip or other controller chip on the logic board or other device within the computer.
Make certain that you have properly installed the iMac Firmware update prior to installing Mac OS 8.6 update. You will receive an alert message and the installation will be halted if you try to install on an iMac that has not been updated.
You can find the update in the CD Extras folder of the Mac OS 8.6 disc or you can download it from the Apple Software Updates Web site at
http://www.apple.com/swupdates.
A list of all updates for each model of the iMac is available in document 58174 titled "
iMac: When to Install Available Updaters"