PaulS:
Even with Firmata installed, the Processing application needs to define the pin mode for ALL the pins being referenced, telling the Arduino instance that xPin, yPin, and zPin are ANALOG pins.
Hmm, well that fixed mine today, though I swear I had those pins defined when I tried it last time. I guess the simple analog firmatas have the pins automatically set. Thanks!
cefn, does that fix yours, too?