If the xbees are in regular modem mode (not API) they should be transparent as far as the connection goes.
That what I though.
You just need to make sure the baud rate is the same on everything.
I start to reset the default settings : Receiver (connected to an Arduino) : ATRE, ATMY1, ATWR Sender : ATRE, ATMY2, ATDL1, ATWR
The ATID is the same by default, so it should be ok. When I use these settings with 2 arduinos, it works. And I've tested settings succesfully with direct connections between XBees. I don't understand. May be Firmata is waiting special messages to be start...
Jerome