A firearm includes a frame, a slide, and modular sear and trigger mechanism housings. The frame has fore and rear receptacles integrally formed therein, e.g., upwardly open compartments accessible from the top of the frame. The modular housings house one or more operational components of the firearm, e.g., trigger and sear components, and are configured for "drop in" insertion into the receptacles. Each housing also includes a set of integral slide rails. For assembly, the operational components are attached to the modular housings, and the housings are dropped into the receptacles and secured to the frame. The slide is operably positioned on the slide rails for reciprocating movement thereon. The frame is provided without integral rails and other features for supporting the operational components. Instead, the rails are provided as part of the modular, drop-in sear and trigger mechanism housings, which also support the operational components.