I have a TPMS tool (yes I'm a tire geek) which allows me to connect with each sensor directly, not through the car's systems. It will instantly tell me the pressure, temperature, serial number, and battery condition of each sensor when I read them. But also this is a $500 tool which most people don't have - but most shops do.
In this case I would return to the Firestone store and ask them to use their tool (assuming they have one) to verify sensor condition. It takes a technician about 5 minutes to do the whole car and they can just do it in the parking lot. I would hope they'd be willing to check that out since the problem started when you brought your car there.
-Bob C.
