I’m not sure what you are attempting. SerialFirmata is used to control serial peripheral devices attached to an Arduino board. You would not need to modify that code. If your idea is to create a simple serial control protocol to communicate with another board the servos are attached to and is running the Servo library, then you can use SerialFirmata to accomplish this. See these two examples: https://github.com/soundanalogous/Breakout/tree/master/examples/serial. You can use this to read and write commands over a serial connection to another Arduino.