In the verbose output when uploading you should see a line somthing like this:
avrdude.conf -v -v -v -v -pattiny85 -cavrispv2 -P\.\COM2 -Uflash:w:C:\Users\EC\AppData\Local\Temp\build7624827821914026099.tmp\Blink.cpp.hex:i
So you could substitute the blink.hex whit the firefly.hex just to be sure that the command line is right