If Firestone didn't give you the road force numbers the first go around, what makes you think they will the next time?

Find a shop that can/will balance the tires and give you the road force numbers. Stand next to them and watch the screen if you have to. The tires are already on the wheels...it's at least worth a shot instead of going back and forth.

Not to mention, your old tires are probably long gone anyway. What are you expecting for Costco to put back on there after you try and return the current set?