There are many synonyms that can be used instead of "firing on." Some of these include shooting, targeting, barraging, bombarding, assaulting, attacking, striking, and hitting. All of these words describe the act of intentionally directing some kind of weapon or force at a specific target. When describing military or combat scenarios, these synonyms can add color and variety to your writing. You could say that a group of soldiers were attacking a fortified position, or that a ship was bombarding a coastal city. Each of these words has slightly different connotations, so it is important to choose the one that best fits the situation you are describing.