Welcome to the forum. Yes, Firefly is a Hertz brand. It was started a few years ago when Hertz had to divest its interest in Advantage. It started overseas then came to USA. Firefly has been in SLC a few years. I've not rented so can't give you firsthand.
Most of the majors has a deeply discounted brand. Firefly is now Hertz. While it's owned by Hertz, you can't expect the same service. For sure it's legit. But generally expect a lower level of service, at the counter and on the road. I believe it does operate out of the actual terminal in SLC, which is a step above its price-range competitors Fox and Advantage.
It does operate out of the terminal. I hung out right by there for almost three hours this week waiting for my pre-booked Budget rental. For what it’s worth, they did not seem to have any of the same issues (line was moving and people were getting cars without a wait) that many were having with Budget that day. I actually tried to get a car from them but they did not have what I needed.
My little brother has used them in SLC and recommended it for the rate so it's legit. A friend and I are both about to rent from there this weekend so will update :-D I had to book through Expedia. Apparently, you can't book through a Firefly site, or at least I couldn't.
UPDATE: the Rental experience was great. They were friendly and gave me a nice upgrade for a minimal upcharge; HOWEVER, there were issues after that. I called to extend my rental. They said there was a problem with my reservation and I needed to just keep the car an extra day and the rate would be the same. Went to return the car, not only did they try to charge me $40 over for the extra day, but they tried to charge me $40 over for every day. Should have been ~$200 for the whole rental and she was charging me over $400. I had to get my paperwork to prove they were charging me too much. I still got overcharged for the last day. I am currently trying to write to them, but I cannot find an email address and the social media links on their web site are fake. Run, run, run away from Firefly!!!
Thank you! That link works now but when I was trying to book a car it didn't. I wish it had. If I hadn't booked through Expedia I might not be having the issues I've described in my update :(
Just be very careful with this car rental service. I got here a half an hour early, and your rental cannot start early. Even if you’re 15 minutes early you have to wait. It’s ridiculous. Additionally make sure that you fill up the gas because they charge $10 a gallon for filling it up.Edited: 11:00 am, August 06, 2018
The worst car rental experience I've got,