Well I have two T/C rifles and the frizzens on them are just fine. I have shot hundreds of shots off them and they still spark excellent. Granted when the frizzen gets soft their sparking ability decreases. If you get a Thompson Center, remember they have customer service next to none. I would venture to guess that in the event the frizzen on the firestorm went soft they would replace it with a new one.
HOW TO HARDEN A FRIZZEN
These are the instructions I was given on how to harden a frizzen. I have never done it, attempted it, or seen it done. In fact I would just replace the frizzen with a better frizzen IF the original one failed. Perhaps you can make sense out of the instructions.
Wipe the face of the frizzen with oil and press it into the casenite ( a chemical powder element I am guessing) to get a good cover, get a bucket of cold water, some tongs if it's pistol sized and clamp it in your vice face up.
Apply heat to get it up to an orange glow, sprinkle on more casenite and cook for a minute or two.
If the frizzen is big, undo the vice and allow the frizzen it to fall in the water. If small use the tongs and hurry it into the water with a swirl.
If it doesn't pop when it hits the water then it wasn't hot enough, so warm it up and try the steps again...
I hoped this helps. Really, my flintlocks I purchased (other then my Lyman) were used. They were shot before me, and I have shot them a great deal. I still and getting excellent spark off them. I think you might be too concerned over the frizzen hardness, but then one never knows..