Fire Sword is a combo ability available in Kirby: Squeak Squad. Kirby gets this by combining the Fire and Sword abilities. It is similar to the regular Sword ability, but the elements of the attacks are all changed to fire, meaning it can light candles, burn grass, and light fuses. It turns into a normal sword underwater, but when Kirby jumps out of the water it turns back into the Fire Sword.
Curiously, along with Ice Sword and Thunder Sword, the flavor text of this ability says it can't be used in water, when the truth is that it actually turns into a regular sword underwater, but it most likely means its fire properties cannot be used underwater.
Rather than cutting grass like Sword normally does, Fire Sword burns it instead.