Hello, just wanted to share my scenario in case it helps.
I hooked up my Firewave today after a long break from it. I did check online (Google) to see if I might run into some issues as I am now running 10.6.2. Currently the software and the unit is working fine on my machine and I did verify all the channels/outputs are working properly.
However, I did run into a frustrating problem that stopped me in my tracks at first - until I remembered to use Apple's "AudioMIDI Setup.app" in the Utilities folder. One must select the Firewave in the list of output/inputs/and devices in the left side of the window Then, at the bottom where the "Plus and Minus" signs are for adding and deleting, click on the "Gear" icon and select "use this device for sound output". Now, once you select the device you still have to tell it to output 2 channels, 6 channels, or encoded in the AudioMIDI app and you can also select 2, 4 or 6 channel surround in the Firewave app!
It's frustrating that there are numerous places to set how many channels to use.
(BTW, I think I recall reading on Griffins website a long time ago that you really don't need to use the Firewave App.)
Anyways, many places to change settings - at the Player level (2 places for DVD), the Third Party App level, and Apple Utility and System Preference level. Then you get in to the file level with AC3 passthru, Dolby Whatever, Mutli Channel, blah, blah. It should be easier, right?
As far as apps crashing, the usual suggestions - delete preferences or everything and re-install. Something is messed up as it starts up. I don't think the Firewave is going to cause a kernel panic or cause apps to freeze, but I could be wrong.
So, I'm using Snow Leopard and Griffin's Firewave and it's working. I'm thinking it should work for you too. I will say also that I wished I had bought two units!
Oh, you did say you need 5.1 and if your machine (lots do) has an optical output it should support 5.1 on that too. But again you must select "digital output" in the various places. You may have to use Perian to ensure that you are sending 5.1 and not Stereo to your optical port because of QuickTime issues.
Good luck