I see, I was confused by the PC in the middle.
The Arduino has an (virtual) serial port via the USB bus. It is used to upload sketches and to show messages for debugging and also to control the Arduino.
Since the Arduino has some tasks to do (like the servos), I think you can not use firmata on top of that.
http://firmata.org/wiki/Main_Page
The program on the PC has to use the (virtual) serial port.
Some use Python, or Java, but you have to program it in the C++ code.
You can make your own communication protocol.
You can make the communication readable, so you can use any terminal program to check upon the Arduino.
The Arduino side of the Serial, start here, Serial - Arduino Reference
Is this the answer you were looking for ?