You stir up an interesting idea by mentioning FireWire. Why don't you put your iMac into target disk mode, connect it to the iBook, and boot the iBook off of the iMac? Ethernet is already configured correctly on the iMac so it may be good troubleshooting to see if that makes the iBook's ethernet spontaneously start working.
They are different machines so if it still doesn't work, the test is inconclusive. But it would tell us something if ethernet does work in this setup.
Are you using a crossover ethernet cable or a patch ethernet cable? Try both. Either should work but you never know.