It appears that the included version of FirmataPlus cannot communicate with the Mega 2560.
A workaround is to either use StandardFirmata or the older version of FirmataPlus
The Uno and Leonardo does not exhibit these issues.
There is a known issue when using the avr watchdog timer on the Mega 2560. The solution is to use either StandardFirmata or the older version of FirmataPlus as mentioned above.
It appears that the included version of FirmataPlus cannot communicate with the Mega 2560. A workaround is to either use StandardFirmata or the older version of FirmataPlus
The Uno and Leonardo does not exhibit these issues.