"Fire" isn't an actual Magic keyword or ability—there's no rules mechanic by that name. You're likely thinking of something else, since fire is purely a flavor/color-identity concept tied to red cards and burn spells.
If you mean firebreathing, that's the informal nickname for abilities like ": This creature gets +1/+0 until end of turn." It's not a keyword either, just a common templated effect. The nuance people miss: the boost lasts only until end of turn and can be activated any number of times if you have the mana, including in response to blocks for combat tricks.
If you actually meant a real keyword—like Firebreathing, First strike, or a card with "Fire" in its name (e.g., Fire // Ice)—clarify which, and I'll break down its rules and Commander relevance precisely.