Stein's molecules became unstable after Ronnie "died" or was separated from his body and he began to die. That was the reason for the team trying to find a replacement for Stein to join with, Jax ended up being compatible. If Stein died, Jax would go on and live a normal life, but if something happened to Jax they would have to look for another person who was compatible with Stein so he could survive.
-People should not be afraid of their governments, governments should be afraid of their people. V4V

none of them would die because stein never died when ronnie did
"sir, sir, i gotta check and see if you've soiled yourself, I'll get to you in a moment, sir!"

Nothing would really happen aside from no more Firestorm. I don't believe Jax would otherwise be affected; Stein would die without fusing with Jax occasionally, but I don't think it was ever even suggested that Jax would die without Stein.

As long as he stays out of churches and away from elem schools he should be fine.
Where are the Asian characters DC CW? #MoreAsians

What happens to Stein if Jack gets killed in a drive by?

I think Stein is the one who requires to recombine with someone from time to time for his molecules to be stable. Not Jax, so he would have a normal life I guess.