Fire Sword is a mix 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, the fire returns.
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.
In Stage 1 of Jam Jungle, there are Ability bubbles for both Fire and Sword hidden in tall grass in the same room together, allowing the player to mix them and obtain Fire Sword when visiting the stage with the Sword Ability Scroll obtained (either by revisting it in Story Mode or doing the stages out of order in Extra Mode).
Stage 2 of Secret Sea also features Sword and Fire Ability bubbles in the same room together, allowing them to be mixed for 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.