An indexoutofrange exception is raised when setting A5 on Arduino Uno to 
digital input or output.

I've traced the exception and came up with a solution. The problem is caused by 
the Handle(DigitalMessage message) function in EasyFirmata.cs

Modified code that works:

        public void Handle(DigitalMessage message)
        {
            var pinStart = (byte)(8*message.Port);
            for (byte i = 0; i < 8; i++)
            {
               int index = i + pinStart;
               if (index < Pins.Count) Pins[index].CurrentValue = message.PinStates[i] ? 1 : 0;
            }

            OnNewDigitalValue(message.Port,message.PinStates);
        }