The heck is Firestorm gonna do to Kyle though? Sure he can manipulate all kinds of elements and matter but Kyle can counter anything he does. Just better.
And without the good scientist in his head telling him what to do he's not very proficient. Which is why he's learning chemistry in college.
It's not an element he recognizes. We're not even sure its an element period. Green Lantern's ring cannot be altered or changed. Even Mxy. failed to do such a feat.
He can't. He can only manipulate matter that is Earthly. Anything on the periodic chart.
He can create kryptonite however he studied it with the good doctor in his head. Ronnie doesn't not possess the knowledge to use his powers proficiently. Atom or Oracle usually have to bark orders at him on what to do. He's an 19 year old kid with amateur knowledge of chemistry. Which is why he's gonna lose.
GL would just blast him to hell with his ring. He can nullify intangibility like he did with the White Martian. Ronnie would just try to think of how to nix GL. By the time he figures out something he's already unconscious.
That or use the ring to absorb his nuclear energy. He's done in Russia to save a city from a nuclear meltdown.
You're wrong about Firestorm only being able to affect elements he knows of. I don't remember the episode name of Superfriends (Yes, I'm using the show as an example, but it's not the only on there is), but he went to another planet that CLEARLY had different elements and used his powers very efficiently there.
I think a fight between Green Lantern and Firestorm would be an incredible one--both attacking and countering the other's attacks. It could go on for as long as their will power held up.