Well, as a first try, here is the DUE info I put into Boards.h after moving the Firmata folder into the DUE's libraries folder. StandardFirmata wouldn't build because it needs Servo.h (and probably more), but SimpleDigitalFirmata compiled and loaded on to the DUE, and I was able to use the processing Sketchbook/blink/blink.pde to blink the LED (pin 13) on the DUE.
// Arduino DUE
#elif defined(__SAM3X8E__)
#define TOTAL_ANALOG_PINS 12
#define TOTAL_PINS 66 // 54 digital + 12 analog
#define VERSION_BLINK_PIN 13
#define IS_PIN_DIGITAL(p) ((p) >= 2 && (p) < TOTAL_PINS)
#define IS_PIN_ANALOG(p) ((p) >= 54 && (p) < TOTAL_PINS)
#define IS_PIN_PWM(p) digitalPinHasPWM(p)
#define IS_PIN_SERVO(p) ((p) >= 2 && (p) - 2 < MAX_SERVOS)
#define IS_PIN_I2C(p) ((p) == 20 || (p) == 21) // 70 71
#define PIN_TO_DIGITAL(p) (p)
#define PIN_TO_ANALOG(p) ((p) - 54)
#define PIN_TO_PWM(p) PIN_TO_DIGITAL(p)
#define PIN_TO_SERVO(p) ((p) - 2)