I recall from a lonnnng time ago (10.0.x days) that firmware updates had to be done using OS 9. However, I have an iMac G3 500 and I haven't had to update any firmware since then (as far as I can recall). Surely we can't still be required to use OS 9 to update firmware, can we? Hope someone knows better than me. Have you tried searching Apple's support forums for "imac" and "firmware" and perhaps "g3"?