There is no reason to use the official HTC tool at all. I'm surprised it even works. Early tests by other users (incl myself) showed our devices being blocked on the site. Are you sure you have a Verizon HTC M8 as opposed to one for another carrier?
Anyway, with firewater, you shouldn't use it at all.
First, it voids your warranty. But most importantly of all, it does not give you S-off. Only bootloader unlock.
Firewater gives you both S-off and unlocked bootloader as well. There also have been no reports of any issues due to firewater. All it does is give you S-off. There are some legit dev phones out there that also have S-off; firewater just makes your phone equal to those dev ones.