Is there a specific reason you are looking for the Firewire interface? It is slightly better than USB 2.0 but USB 3.0 is a lot more faster.
I recall the WD MyBook used to have this interface - I've used a MyBook Home 500GB sometime back and it had a couple of Firewire ports (for daisy chaining) but rarely used it. I doubt if you still get external drives with the Firewire interface. I believe USB 3.0 may have replaced it.