What if you hold down the option key while starting up?
I seem to remember that just because a drive is Firewire it does not guarantee it is bootable. Chipsets can vary and most go for Oxford chipsets in FW devices because of full compatibility.
Is your eMac capable of booting OS9? One of the 1GHz models couldn't, any after that couldn't, and the ones that could need the special Install Disc that came with them.
At the Apple Icon at top left>About this Mac.
Then click on More Info>Hardware and report this upto *but not including the Serial#*...
Hardware Overview:
Machine Name: Power Mac G5 Quad
Machine Model: PowerMac11,2
CPU Type: PowerPC G5 (1.1)
Number Of CPUs: 4
CPU Speed: 2.5 GHz
L2 Cache (per CPU): 1 MB
Memory: 10 GB
Bus Speed: 1.25 GHz
Boot ROM Version: 5.2.7f1
This reminds me of the time many years ago when my father and I tried to assemble my twin sons' Christmas tricycles after a few drinks...didn't use the manual...at midnight we looked at the manual!!!
Used the original eMac System Install and Restore disk to restore Classic Support...naturally it worked!