my firefox flame update failed (1.3 -> 2.0).
after update failure, the bootloader no longer into fastboot mode.
IMHO, failed reason is due to multi-directional firefox flame manual.
firefox flame manual should be changed to unidirectional.
firefox flame emergency download tool saved my phone,
run the utilities and retry B2G 184.108.40.206 again, it worked.
Emergency Download Tool is windows only.
If flashing a new build to your phone fails to work, your phone becomes unresponsive, and the phone cannot enter fastboot mode, you can use emergency mode for recovery. A USB cable and the Emergency Download Tool are required to enter emergency download mode. Install this tool and follow the instructions.
PS: after update, power button + volume down button into fastboot mode.