A firing squad was a group that would be designated to carry out an execution. This would be done by having the person to be executed stand against a wall, tied to a post, or something similar and be shot with a rifle, musket, or phaser.
Q and Kathryn Janeway faced execution by firing squad during the Q Civil War in the Q Continuum. However, crewmembers from USS Voyager intervened and saved them. (VOY: "The Q and the Grey")