Booting from 10.3.7, I plugged in an external FireWire drive (ACOM Data Combo FireWire400/USB 2.0). I then ran the combo updater on the drive.

After this, I plugged the drive back into my TiBook 867. It would see the drive, and I could select it to boot from, but it would consistently fail to boot about 1 second into the boot process. This is the exact same failure as I had seen with some FireWire drives and 10.3.6. My suspicion is that the 10.3.7 FireWire drivers are still buggy. I was able to rescue the drive by doing an "Archive & Install" of 10.3.0 then applying 10.3.5 again.

ACOMData's website has no driver updates yet.