Including the library you found is not going to work simply by including it in the sketch. You need to write a wrapper so that Firmata understands how to communicate with that library. This is not so easy. You can find examples in the custom_examples directory: https://github.com/soundanalogous/Breakout/tree/master/custom_examples. Basically look at how the RFID library wrapper works in those examples. You’d have to do something similar to get the Servo library to work.

Likewise, you can’t pass arbitrary assignments such as “x = 12” through Firmata since Firmata doesn’t know what to do with the value. You need to extend StandardFirmata to understand such things, much like in the custom examples I linked to above.

I also recommend looking into using Johnny-Five instead of Breakout because Johnny-Five has a nice Servo animation library that is probably exactly what you need. Johnny-Five is also built in NodeJS so the networking capabilities are pretty much unlimited. You may not find Johnny-Five specific networking examples, but you can look up NodeJS networking examples, of which there are thousands.

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.

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.