I finally found how to upload custom firmware to RFbee using UARTSBee.
I had to set Arduino IDE with :
card type : Lilypad Arduino
processor : ATMega328
Programmer : AVRISP
In this configuration upload baudrate is set to 57600. (if using ATMega168, baudrate switch to 19200)
It’s not really what is said on seeeduino wiki, but till it works I don’t care.
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