Mo. Rev. Stat. § 320.122

Where regulations or prohibitions of the sale of fireworks are adopted by any first class county operating under a charter form of government and which contains a population in excess of nine hundred thousand inhabitants, such regulations or prohibitions shall supersede, as to those matters to which this section relates, all municipal ordinances, rules and regulations within the boundaries of such first class chartered county, but only to the extent such regulations or prohibitions are more restrictive than those adopted by a municipality located within such county.