Other forms: spitfires
Someone who's a spitfire has a fiery or passionate temper. If your sister slams her door dramatically whenever she's angry, you can describe her as a spitfire.
From the early 1600s, spitfire was used for things that literally appeared to "spit" fire, and also for people who figuratively seemed to do the same. The word described both military cannons and fierce, quick-tempered personalities. Today, it's almost always used for someone who's intense and maybe a little bit difficult: "They have to pay the babysitter extra because my little cousins are such spitfires!"