A firearm holster system is described that includes a body configured to receive a firearm mounted with a firearm accessory. The holster includes an engagement member including two rail interfaces: one for detachably mounting the engagement member to the firearm, and another for detachably mounting a firearm accessory to the engagement member. The body of the holster is adapted to receive and detachably secure the engagement member. The holster can accommodate most standard firearms, and most firearm accessories. The holster includes a lockable lever to secure the engagement member to the body of the holster.