In StandardFirmata or StandardFirmataPlus, change these lines:

Firmata.begin(57600);
while (!Serial) {
  ; // wait for serial port to connect. Needed for ATmega32u4-based boards and Arduino 101
}

to:

Serial1.begin(57600);
Firmata.begin(Serial1);

And connect to the TX1 and RX1 pins on your board. You need to use an Arduino board what has these pins. An Uno doesn’t for example but a Leonardo, Due or Mega does. Due and Mega also have TX2 & RX2 which you’d use Serial2 instead of Serial1. TX3 & RX3 use Serial3, etc.