Can I rent a car in Orvieto and drop it off at the Florence Airport?
yes
Yes. I did it, no problem. If you are going to Florence after dropping off the car there is an express bus that will take you to the train station. I believe it is 5€ per person.
Hertz is across the street from the Orvieto train station. We got a Hertz car when renting thru AutoEurope.com at a cheaper rate. There are some Florence city locations for Hertz where the car can be dropped off, like 60 Sansovino.
I avoid airport auto rental locations worldwide as they often come with large taxes and administrative fees.