Is it possible to pick up a rental car at Barcelona Airport El Prat de Llobregat to cross the border to france to do some shopping and return?
Yes. You can find cross border information here:-
To view the CROSS BORDER or FERRY terms and conditions. At STEP2 of our 3 step online booking process, alongside the car photo of your choice, there is a link entitled “IMPORTANT INFORMATION”. Clicking on this link will open an additional smaller window, with the terms and conditions appertaining to that particular car/supplier. Choose then the CAR tab and look under TRAVEL RESTRICTIONS
Try a few different suppliers as each supplier/vehicle type may have a different cross border rule.
Specifically asking about Firefly!
OK, select supplier Firefly with the supplier filter at the left hand side of the page. Then click on IMPORTANT INFORMATION with a Firefly car...then look under CAR > Travel restrictions.
Thank you for your help, I will check.
I took a quick look for you: Firefly allow crossborder to France, additional fee per day 9.08€