Applicants have disclosed an adjustable modular trigger assembly, and a related method, to avoid an unexpected firing of a long gun after an unintentional trigger pull. Applicants' preferred assembly comprises: a modular trigger housing, containing every trigger component, designed to be removably mounted within a lower firearm receiver; a trigger pull adjustment screw in a housing wall; and a sear force adjustment screw in another housing wall. During an intentional trigger pull, the shooter's finger pushes against a secondary trigger until it nestles within a slot of a non-coaxial primary trigger. Continued pulling on both triggers causes a sear to fall off a hammer notch and the firearm to discharge. In an unintentional trigger pull or jostling of the primary trigger, hooked protrusions (within the housing) of the second trigger and hammer engage to block the hammer from fully rotating, thereby avoiding discharge of the firearm.