A surgical tool assembly, comprising: a shaft assembly comprising: an axially movable closure member; and an axially movable firing member selectively axially movable between an unfired position and a fired position and wherein said surgical tool assembly further comprises a surgical end effector comprising: a first jaw; a second jaw comprising a mounting portion including a pair of mounting walls configured to movably engage a portion of said first jaw to movably support the second jaw on the first jaw, said mounting portion defining a cam area configured for engagement by said axially movable closure member to move the second jaw from an open position to a closed position relative to said first jaw, said mounting walls defining an open topped parking area therebetween for accommodating said firing member therein when said firing member is in said unfired position; characterised in that the surgical tool assembly further comprises means for at least partially covering at least a portion of said open topped parking area. a shaft assembly comprising: an axially movable closure member; and an axially movable firing member selectively axially movable between an unfired position and a fired position and wherein said surgical tool assembly further comprises a surgical end effector comprising: a first jaw; a second jaw comprising a mounting portion including a pair of mounting walls configured to movably engage a portion of said first jaw to movably support the second jaw on the first jaw, said mounting portion defining a cam area configured for engagement by said axially movable closure member to move the second jaw from an open position to a closed position relative to said first jaw, said mounting walls defining an open topped parking area therebetween for accommodating said firing member therein when said firing member is in said unfired position; an axially movable closure member; and an axially movable firing member selectively axially movable between an unfired position and a fired position and wherein said surgical tool assembly further comprises a surgical end effector comprising: a first jaw; a second jaw comprising a mounting portion including a pair of mounting walls configured to movably engage a portion of said first jaw to movably support the second jaw on the first jaw, said mounting portion defining a cam area configured for engagement by said axially movable closure member to move the second jaw from an open position to a closed position relative to said first jaw, said mounting walls defining an open topped parking area therebetween for accommodating said firing member therein when said firing member is in said unfired position; a first jaw; a second jaw comprising a mounting portion including a pair of mounting walls configured to movably engage a portion of said first jaw to movably support the second jaw on the first jaw, said mounting portion defining a cam area configured for engagement by said axially movable closure member to move the second jaw from an open position to a closed position relative to said first jaw, said mounting walls defining an open topped parking area therebetween for accommodating said firing member therein when said firing member is in said unfired position; characterised in that the surgical tool assembly further comprises means for at least partially covering at least a portion of said open topped parking area.