Fire Sword is a combo ability available in Kirby: Squeak Squad. Kirby can obtain this ability 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 set off fuses. It turns into a normal sword underwater, but when Kirby jumps out of the water, it turns back into the Fire Sword.
In the trailer for Kirby: Star Allies, Fire Sword is shown to have returned as a Mixed Ability. The blade is now given a curved design, no longer sharing its shape with Sword. The hat, however, still retains the same design. The sword is now capable of creating a long trail of flames when the Sword Dive move is executed. Blade Knight is also hinted to be capable of using the Mixed Ability as a Helper.
Kirby slashes the enemy with his sword.
B + B
Kirby swings his sword very fast in slashes at different angles.
B + B + B
Kirby attacks with lightning-fast sword slashes that are nearly impossible to see.
Up + B
Kirby jumps in the air and does a flip, then he falls sword outstretched. This move comes from Cutter.
Kirby swings the sword quickly in midair, damaging all enemies that come into contact with it.
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.