I dont' have any experience with the firestone product, but haven't had any issues with my Airlift, after being installed for 5+ years they are still going.
I have had the Firestone Coil-Rite for about 2 years now and haven't had any issues with it. It has been a GREAT improvement to the rear suspension. I have pulled a car trailer with a mid size car on it and didn't have any issues at all.
They are pretty much the same thing with the exception of the color of the air bags. Airlift's are red and the Firestone's are blue. I would go with the one I could get for the cheapest price.