Board Thread:Character Match-Ups/@comment-26921356-20180529030828/@comment-33314856-20180608133253

Found something better: Atom Eve (Image Comics)

Firestorm's main power: "the ability to rearrange Molecular or Particle structure of any substance into most anything else"

Atom Eve's main power: "manipulate matter at the sub-atomic level by sheer will power. Organic (not animal life, however) and inorganic matter can be changed into anything she desires"

Atom Eve's powers are mental, Firestorms are physical. This is the only matchup where Firestorm can't immediately win by turning their opponents costume into liquid death or something.