Can an eMac boot from a USB optical drive connected via a FireWire adapter?
I believe you’ll have better luck with USB drives that are not optical. FireWire optical drives that supported booting in this era were expensive, so see if you can score a drive that you know used to boot. Even three at scrap prices are often salvageable with DIY skills.
I’m almost certain we booted these Macs from USB drives in the shop back in the day. It wasn’t “supported” but it worked most of the time.