FireFly seems to be in a class by itself, at the very bottom. (And I have direct experience with virtually every other USA rental car brand.)
As others noted the clerks are trained to use FUD in a high-pressure up-sell conversation as you rent. They will tell you that those rentals have no insurance. That if the rental needs repair after your use, you will liable for the list-pice rental fee during the repair period, etc.
FireFly cuts costs by leasing the furthest away, cheapest parking area for their fleet. They have no personnel in the car area for check out or check in. Their Lost-and-Found is a black hole of no-response. Don't count on ever seeing again anything you leave behind.
Finally, and most egregious, is the condition of their fleet. Our Dodge Avenger (perhaps a piece of trash by itself - way underpowered and terrible visibility) had over 50k miles on it, pulsating brakes and handling that caused us to keep in the slow lane (out of alignment?) and never keep-up-with traffic in Arizona's 80+ MPH Interstate speeds. Ugly.
The savings of a couple of dollars per day was not worth it. Will not rent from these guys again and suggest avoiding them.