Erni:
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
I think you may have figured it out!!! I found that line and typed this into the AVRdude command line:
avrdude -v -v -v -v -pattiny45 -cstk500v1 -P/dev/cu.usbmodemfd121 -b19200 -Uflash:w:firefly.hex
...and it looks like IT WORKED!!! The only thing to do now is remove the ATtiny45 from the breadboard, solder it to the surfboard, and test it! I'll report back. Thanks!