I have the standard firmata loaded onto my Arduino with a light sensor on my breadboard hooked up to the '0' analog output.
Analog pin 0 is not an output. By loading firmata on the Arduino, you've given it a lobotomy. It does NOT send data to Processing on its own. It only responds to requests for data.
void draw(){
}
So, you never ask the Arduino about the pin, so Processing does nothing. Not surprising.