What kind of computer/OS are you using?
As cool as the standard firmata is, if I were to do something like this I would probably use MIDI or maybe OSC to make it more flexible and fit it in with my larger plans. I had fun playing with the circuit bending / Max / arduino combo 10 years ago but these days I am considerably more deliberate.
If I were you, I would grab an arduino teensy 3.2 and set it to class compliant midi mode, plugs in to usb and automatically shows up as a midi device. Works with just about anything, plenty of info / tutorials and examples.. overall if you are into arduino and synthesis you should get a teensy.. they are the bomb, I prototyped my omnimod eurorack module using teensy 3.1. https://youtu.be/cBUt_ciNYW8