Firewire Target Disk Mode is not dependent on any OS - it is a hardware function, available on all machines which support it. This includes all machines with built-in firewire except for two - the B&W G3 desktop models and the G4 (PCI graphics) models (those can be the Host machine, but not the Target machine).
What OSX has that OS 9 does not have is a software means of directing the machine to use FwTDM.
The article that Tom provided a link to describes how to direct a machine to start as the Target machine in FwTDM using a startup keyboard command.
The steps are (copied from Article #HT1661) -
1. Make sure that the target computer is turned off. If you are using a PowerBook or iBook as the target computer, you should also plug in its AC power adapter. 2. Use a FireWire cable (6-pin to 6-pin) to connect the target computer to a host computer. The host computer does not need to be turned off. 3. Start up the target computer and immediately press and hold down the T key until the FireWire icon appears. The hard disk of the target computer should become available to the host computer and will likely appear on desktop. (If the target computer is running Mac OS X 10.4 Tiger, you can also open System Preferences, choose Startup Disk, and click Target Disk Mode. Then restart the computer and it will start up in Target Disk Mode.) 4. When you are finished copying files, drag the target computer's hard disk icon to the Trash or select Put Away from the File menu (Mac OS 9) or Eject from the File menu (Mac OS X). 5. Press the target computer's power button to turn it off. 6. Unplug the FireWire cable.
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It is usually better to set the older Mac to be the Target machine and the newer one to be the Host machine, provided the older machine is Target capable.
