Firienfeld
From Tolkien Gateway
Firienfeld was how the Rohirrim called the flat upland of Dunharrow; a large grassy area for the encampment of soldiers and refuge-seekers.
Etymology
Firien represents Old English firgen, "mountain"; -feld, means "field" and Tolkien left it unmodernised; the Firienfield of the Index is in error.[1]