I have a Memorex external DVD burner. It can be connected with with USB or Firewire. It works fine in USB, but DVDFab will not recognize the drive if it's connected via Firewire.
I should add that my PC (win xp) sees the drive just fine if it's connected by Firewire, and that I've used ImgBurn to burn dvds via firewire. This seems to be a DvdFab-specific problem.
In case you're wondering why it matters, I have a free firewire port and no free USB ports.