Jeroen,

If the caliper is seized, you are out of luck. GL trying to prove Firestone is at fault.
Calipers seize b/c of rust and/or excess heat. If the pad / rotor aren't working correctly or the pad is really worn down (metal on metal), then the heat builds and the caliper suffers.

If you're handy, get a caliper at a junkyard and put it in yourself. If you're not, DON'T take it back to Firestone IMO.

Those chain shops (Meinke, Firestone) can be really bad news--unethical, incompetent. I would go to a local independent shop.