a extended version of Firmata is available at https://github.com/robvanlopik/Firmata. The main changes are:
added support for i2c, stepper and encoder
limited thread safety
announcements for change of digital input and end of stepper motion
access to capability request and analog mapping
supports configurable Firmata
easier to add new devices (accelstepper and one-wire are on my list)
added a description of the API
I tested with different Arduino boards from a Windows system through a USB connection.
I hope you will take a look at the code and see whether it is worthy to replace the present version.
cheers
Rob van Lopik
https://github.com/robvanlopik