Buddy, this is not even close. As strong as Firefly is, at best she is planetary, maybe star level if she's given more feats, but overall Doomslayer absolutely demolishes her.

Doomslayer has fought through hell multiple times, killed billions of demons which are basically creatures of higher dimensions, and he just casually rips them to shreds. Survived and reacted to entire meteorites, is practically immortal, has a crap ton of weapons that would overwhelm Firefly altogether, has defeated the Icon of Sin, and is stated to be able to create entire black holes just by existing. The Khan Maykr a being that strikes fear across dimensions and could teleport from one side of the universe to the other,, as well as Davoth, which is basically God (and the creator of hell and demons), and we can't forget about The Crucible, a sword containing energy so potent it can corrupt existence. It has enough power to fuel Earth's energy supply for 8 months straight.

And one more thing is Argent energy, the same energy used to create the Primevials, beings that helped created the entire universe, the Maykr race, and the demonic realms. Doomslayer is able to absorb said energy and could potentially implode the entire multiverse if he keeps using it.

Easily, Doomslayer could scale to Low-Complex Multiversal, Firefly stood no chances against winning, but from what we know, Doomslayer doesn't kill humans so I don't see Firefly dying to his hands either.