I think her skills at manipulating matter is better. Firestorm transmutes matter or molecules or something like that. She could probably do whatever Firestorm can do and then some. Not to mention she has very powerful psionic ability, nearly as strong as Thanos, and a very powerful telepath, in which she was one of two people that I know that broke Adam Warlock's nearly invincible telepathic shielding. I think Exodus was the other person. I think Sersi takes this.