The Collaborative International Dictionary of English v.0.48:

firebrick \firebrick\ n.
   a brick made of fire clay, used for lining e.g. furnaces and
   chimneys.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

firebrick
    n 1: brick made of fire clay; used for lining e.g. furnaces and
         chimneys

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "firebrick":
   adobe, ashlar, biscuit, bisque, bowl, brick, bricks and mortar,
   cement, ceramic ware, ceramics, china, clinker, concrete,
   covering materials, crock, crockery, enamelware, ferroconcrete,
   flag, flagstone, flooring, glass, jug, lath and plaster, masonry,
   mortar, pavement, paving, paving material, plasters, porcelain,
   pot, pottery, prestressed concrete, refractory, roofage, roofing,
   siding, stone, tile, tiling, urn, vase, walling