Does not sound like you did it wrong.
Today I purchased an 800/400 firewire to connect my new Mac Mini Lion to my old Mac OS X 10.3.9 in effort to view my old computer hardrive from from my new Mac Mini.
I followed the process starting my old computer pressing and holding the "T" key waiting to see the floating firewire symbol and then connecting the two computers with the firewire.This should have caused my old computer to open up the desktop, but did not. The firewire continued to float around the screen and the desktop never opened up.
Next I booted my Mac Mini which opened up but the mouse was frozen. I then toggled between both computers in effort to free up the mouse to no avail.
I then disconnected the firewire which freed up my mouse and then re-attached the firewire hoping the firewire would now work which it did not.
Does anyone know what I may have done wrong or tell me what I need to do to be able to see my old mac hardrive using my new Mac Mini?