What firmware? Where did you get it? What version number or file name?
Also, do fastboot getvar all and post the results (delete IMEI and serial number before posting) so we can see specific info on the device.
You don't need s-off to go back to stock. Only current working method to s-off the phone is with sunshine app ($25 US) but it is overkill, as I mentioned you don't need s-off.