All the FireWire readers I've ever seen are CompactFlash-only.
I'm not sure if this is a marketing thing ("Only pros would shell out for a FireWire reader, and they'll all use CompactFlash") or an engineering one ("It's easier to make a CompactFlash card look like an external disk").
It would be nice to have a multi-format FireWire reader, though, seeing as how my computer is USB 1.1 only (with no way to upgrade to USB 2.0), while my camera takes SD/SDHC.