What it does
"Fire Cross" isn't a real Magic: The Gathering keyword or ability. It may be something you've misremembered, a fan-made term, or confused with another card or mechanic.
If you're thinking of a specific effect, a few possibilities: you might mean Crossfire type effects, a particular burn spell, or a card with "Fire" in its name like Fire//Ice (a split card). There's no official keyword by this name in the game's rules.
If you can share the exact card name or where you saw "Fire Cross," I can give you an accurate rundown of how it works and where it matters in Commander. I'd rather point you to the correct ruling than invent mechanics for a term that doesn't exist in the comprehensive rules.
Iconic cards with Fire Cross
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