Howdy. It seems like a lot to register and make a forum post for something so small, but…
The START_SYSEX message type is incorrectly represented as SYSEX_START.
I know it’s minor, but it threw me for a little while, and I’m fairly experienced with protocols(and isn’t it always the minor bugs that cause the most pain?). I imagine your average newbie wanting arbitrary message data sent to their Arduino would be completely lost.
One other suggestion: Helpful if some kind of checking were in order to see if a thread-post is pointing back at the arduino.cc (or wikipedia, or any of the “normal” pantheon of sites surrounding arduino) so that a correction post doesn’t get discouraged by the “no links until you’ve posted a normal message” constraint. I found that I can’t actually link to the Firmata docs because of it.