A surgical instrument comprising: a surgical end effector comprising: first and second jaw members wherein application of a closure motion to at least one of said first and second jaw members results in the movement of said at least one of said first and second jaw members to a closed position relative to the other of said first and second jaw members; and a firing member operably supported in one of said first and second jaw members for axial travel therein in response to an application of a firing motion thereto and wherein said surgical instrument further comprises: a shaft assembly defining a shaft axis; an articulation joint coupling said surgical end effector to said shaft assembly for selective articulation relative thereto about an articulation axis (B-B) that is transverse to said shaft axis upon application of articulation motions to said surgical end effector; a flexible firing beam extending through said articulation joint to interface with said firing member in said surgical end effector to apply said firing motion thereto; and a movable firing beam support assembly configured to movably contact said firing beam on each lateral side thereof as said firing beam flexes about said articulation joint, characterized in that said movable firing beam support assembly further comprises: a first compression plate positioned against a portion of a first lateral side of said flexible firing beam; a second compression plate positioned against a portion of a second lateral side of said flexible firing beam; and a support boss assembly rotatably and laterally movable relative to a support axis that is transverse to said shaft axis, said support boss assembly comprising: a first support boss pin in contact with said first compression plate; and a second support boss pin in contact with said second compression plate. a surgical end effector comprising: first and second jaw members wherein application of a closure motion to at least one of said first and second jaw members results in the movement of said at least one of said first and second jaw members to a closed position relative to the other of said first and second jaw members; and a firing member operably supported in one of said first and second jaw members for axial travel therein in response to an application of a firing motion thereto and wherein said surgical instrument further comprises: first and second jaw members wherein application of a closure motion to at least one of said first and second jaw members results in the movement of said at least one of said first and second jaw members to a closed position relative to the other of said first and second jaw members; and a firing member operably supported in one of said first and second jaw members for axial travel therein in response to an application of a firing motion thereto and wherein said surgical instrument further comprises: a shaft assembly defining a shaft axis; an articulation joint coupling said surgical end effector to said shaft assembly for selective articulation relative thereto about an articulation axis (B-B) that is transverse to said shaft axis upon application of articulation motions to said surgical end effector; a flexible firing beam extending through said articulation joint to interface with said firing member in said surgical end effector to apply said firing motion thereto; and a movable firing beam support assembly configured to movably contact said firing beam on each lateral side thereof as said firing beam flexes about said articulation joint, a first compression plate positioned against a portion of a first lateral side of said flexible firing beam; a second compression plate positioned against a portion of a second lateral side of said flexible firing beam; and a support boss assembly rotatably and laterally movable relative to a support axis that is transverse to said shaft axis, said support boss assembly comprising: a first support boss pin in contact with said first compression plate; and a second support boss pin in contact with said second compression plate. a first support boss pin in contact with said first compression plate; and a second support boss pin in contact with said second compression plate.