A lightened, molded plastic closure exhibiting enhanced strength includes a top wall portion, and an internally threaded, depending annular skirt portion. The interior surface of the skirt portion, below the closure thread, defines an array of alternating grooves and lands, which together define a plurality of circumferentially spaced, vertically extending strengthening ribs on the inside surface of the skirt portion. The skirt portion thus exhibits desirably enhanced column strength, thereby facilitating efficient formation by “stripping” from an associated core pin during molding. In the preferred form, the exterior surface of the skirt portion is provided with a plurality of circumferentially spaced gripping knurls respectively aligned with the grooves defined at the interior surface of the skirt portion.