I just push a tone branch: https://github.com/firmata/arduino/tree/tone. Have only tested this in Arduino 1.0.5 so far. Replace the Firmata directory in Arduino 1.0.5 with the contents of the tone branch. Then load File -> Examples -> Firmata -> ConfigurableFirmata to your board.

Here is Breakout code to test it with: http://breakoutjs.com/examples/test/tone_simple.html