That got it working, thanks Paul!
If anyone else runs into this, on OS X the file was here on my machine:
/Applications/Arduino.app/Contents/Java/libraries/Firmata/Boards.h
In that file look for:
...
// Teensy 3.0, 3.1 and 3.2
...
#define IS_PIN_DIGITAL(p) ((p) >= 0 && (p) <= 33)
...
And...