A firing squad was a group that was 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.
In 2153, Trip Tucker feared he and three Denobulans would wind up facing a Xantoras firing squad. (ENT: "The Breach")
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")
In 2376, as Michael Sullivan tried to calm the panic spreading through the people of Fair Haven, he sarcastically asked if they wanted to line up "Katie O'Clare" and the other Spirit folk in front of a firing squad. (VOY: "Spirit Folk")