Personally we haven't used Firefly, but they are the budget sister company to Hertz and the rental fleet is shared as needed.
I believe they are in the terminal now.
Reviews on Yelp are mostly negative. I always take reviews with a grain of salt, usually people with a beef will post a negative review whereas people that had a good or average experience will not bother to post. Kind of skews the results unless the company being reviewed is really exceptional (or really bad) https://www.yelp.ca/biz/firefly-car-rental-orlando-2