The Collaborative International Dictionary of English v.0.48:

Frith \Frith\ (fr[i^]th), n. [OE. firth, Icel. fj["o]r[eth]r;
   akin to Sw. fj[aum]rd, Dan. fiord, E. ford. [root]78. See
   Ford, n., and cf. Firth, Fiord, Fret a frith, Port
   a harbor.]
   [1913 Webster]
   1. (Geog.) A narrow arm of the sea; an estuary; the opening
      of a river into the sea; as, the Frith of Forth. Also
      called firth.
      [1913 Webster]

   2. A kind of weir for catching fish. [Eng.] --Carew.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Firth \Firth\ (f[~e]rth), n. [Scot. See Frith.] (Geog.)
   An arm of the sea; a frith.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Firth
    n 1: English linguist who contributed to linguistic semantics
         and to prosodic phonology and who was noted for his
         insistence on studying both sound and meaning in context
         (1890-1960) [syn: Firth, J. R. Firth, John Rupert
         Firth]
    2: a long narrow estuary (especially in Scotland)

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "firth":
   arm, armlet, bay, bayou, belt, bight, boca, cove, creek, estuary,
   euripus, fjord, frith, gulf, gut, harbor, inlet, kyle, loch, mouth,
   narrow, narrow seas, narrows, natural harbor, reach, road, roads,
   roadstead, sound, strait, straits



U.S. Gazetteer Places (2000):

Firth, NE -- U.S. village in Nebraska
   Population (2000):    564
   Housing Units (2000): 200
   Land area (2000):     0.304447 sq. miles (0.788515 sq. km)
   Water area (2000):    0.005747 sq. miles (0.014884 sq. km)
   Total area (2000):    0.310194 sq. miles (0.803399 sq. km)
   FIPS code:            16935
   Located within:       Nebraska (NE), FIPS 31
   Location:             40.534344 N, 96.605691 W
   ZIP Codes (1990):     68358
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Firth, NE
    Firth


U.S. Gazetteer Places (2000):

Firth, ID -- U.S. city in Idaho
   Population (2000):    408
   Housing Units (2000): 148
   Land area (2000):     0.190681 sq. miles (0.493861 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    0.190681 sq. miles (0.493861 sq. km)
   FIPS code:            27910
   Located within:       Idaho (ID), FIPS 16
   Location:             43.305519 N, 112.183938 W
   ZIP Codes (1990):     83236
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Firth, ID
    Firth