Contrary to many negative reactions, a positive reaction. We had a very good experience with FireFly (via RentalCars) in Barcelona Airport.
When it was picked up on September 16, 2018 it was busy but they work with tracking numbers so you know when it's your turn. The pick up of the car is done in 2 steps. First check the car for (small) damage, write it on the form and then have it signed in the office of FireFly. The car was full of fuel and you pay a fee and service charge as stated in the conditions. In total this was  3.30 +  26.22 + 21% VAT =  35.72. The car we have on October 9, 2018 with a full tank surrendered at the airport (without damage) so the final bill that we received within a few days was and remained totally  35.72. With us you could hand in the car with an empty tank but you could also return it with full tank. We have the car refueled at the airport (there is a pumping station at the airport so refuel first and then hand in the car) We have driven with a split new Seat IBiza at the start there was 5100 km on the counter and at handing 9100 km. We are very satisfied with the service and handling by FireFly.
Andre and Annelies Homan, Almere, the Netherlands.