Always get them to write on your copy of the agreement that there is no damage and the contents of the tank, I always return it full anyway. Verbal is pointless.They will then put a company stamp on it and it's your to keep. I always photograph the car when I leave it as well. Just stand your ground. You have to allow enough time to do this before you go to departures. I always have my own annual insurance and have not been pressurised into taking any additional for a number of year's now.
Hertz and Firefly are basically the same company and I've had problems with Firefly in the past.