Firewire to Air
Have yet to get a definitive answer to this question. How can I hook a Fire-wire only Lacie DVD burner to a new Mac-book Air?
Buy a 27-inch Thunderbolt Display, plug the DVD burner into the Firewire port on the display, and use the burner when the Air is docked to the display.
Otherwise, you don't. The Air doesn't have a Firewire port, other Thunderbolt-to-Firewire adapters aren't out yet, and USB-to-Firewire adapters don't exist.
If your older Mac is still operational, another possibility might be to have it share the DVD drive over a network with the Air. That might just get you read access (e.g., for installing software), as opposed to the ability to burn CDs and DVDs.
pretty soon you will be able to buy things like the "Sonnet Allegro™ FW800 Thunderbolt™ Adapter. Connect a FireWire® 800 device, even a bus-powered drive, to any computer with a Thunderbolt port" and there will be similar thingies for ethernet, SD cards etc
but right now- what the previous post said