'syþðan he hire folmum æthran' - hire refers back to duru, a feminine noun.
To discerning eyes, the manuscript reads hran here, though some can only discern an. M-R emend to onhran, 'touched' (translate 'after he [Grendel] touched it [the door] with his hands'). Wrenn, Swanton and Jack prefer gehran, and ASPR (followed here) æthran. Both verbs are similar in meaning to onhran.