I had the same problem a couple of weeks ago. I wasn't able to fix it, but here's a work around: instead of installing one of the standardFirmata programs on your arduino install either SimpleAnalogFirmata or AnalogFirmata from the examples folder. (Only one of these will work, but I can't remember which one it is.) I tried comparing the only working version to the standardFirmata, but couldn't figure out the crucial difference to make it work.
Let me know if trying this works for you too. This may be an issue we need to make the arduino team, or firmata developers aware of.