I've got Earthlink DSL and know a bit about this.

First, Ethernet is definitely the way to go and I agree that it's unlikely that FireWire is coming to DSL modems.

Most DSL accounts these days connect using PPPOE (Point to Point Over Ethernet). OS 9 needs some additional client software to connect but OS X supports PPPOE natively.

Set-up is pretty straight-forward. The user enters their PPPOE ID and password into the software and connects.

Hope this helps.