I noticed this same sort of problem with a FW-connected CD (I have the pricier Que drive). Here's what you might try and why. Too bad your post was ignored for over two weeks. I just now got this working myself. I think Apple goofed.
First it's the Mac's job to recognize both CD drives in the Sound C-Strip and the CD- C-Strip modules. There should be menu items for both devices within these menus, and if you open the 9.1 sound control panel, under the input tab you should see two listings for "CD": one for the internal DVD (ATAPI, right?) and another one for the FW-connected device. How to get it to work?
Since your Fire drive probably came with Toast, you're entitled to an extension called "Toast FireWire Support" version 1.0.5. (get it at roxio.com) With this extension running (that is, restart the iMac with this item in the Extensions folder), Toast can mount the CD and add the drive to the control strip menus and the sound control panel.
One more thing . . .
Since the CD plays okay from the Fire drive anyway, check that your sound input in the Sound control panel is switchable from the Fire drive to the internal DVD. You can only have one "program source" playing at a time. And just now I crashed my 500 MHz G4 because I tried to play two CDs at the same time and toggle the sound back and forth between them.