Reader's workaround for Snow Leopard Firewire problems - My main work machine is still running 10.5.8, but have 10.6.1 running on other Macs and I've not seen any FW issues like this so far, but only used a FW HD dock with them once. (As far as external drives, I've primarily been using an eSATA JMB360 Expresscard connected eSATA HD dock with the 10.6.1 MBP.)
" Firewire and Snow Leopard
Since installing Snow Leopard I had problems with two Firewire DVD burners, one would not mount disks and the other exhibited read problems. They worked fine when connected by way of USB and had worked fine under Leopard. My Firewire hard drives worked OK, although others have had the problem with hard drives as well.
The problem is with a Firewire extension in Snow Leopard IOFirewireSerialBusProtocolTransport.kext
Replacing it with the extension from 10.5 seems to solve the problem. (but does that work w/64bit kernel boots?)
Some people have replaced all five Firewire extensions with those from 10.5. Results have been mixed. For some it is successful, for others the thing that works is replacing only the one extension.
One caveat. Be sure to repair permissions BEFORE restarting. If you don't, for some reason the replacement extension(s) may not be recognized despite repairing permissions after a restart and then restarting again.If this occurs you would have to repeat the entire process.
I haven't seen any issues with 10.6 and Firewire yet but could be helpful for someone.