The Collaborative International Dictionary of English v.0.48:

Firewood \Fire"wood`\ (f[imac]r"w[oo^]d`), n.
   Wood for fuel.
   [1913 Webster]

WordNet (r) 3.0 (2006):

firewood
    n 1: wood used for fuel; "they collected and cut their own
         firewood"

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "firewood":
   backlog, bavin, beam, billet, board, boarding, brush, brushwood,
   clapboard, cord, cordwood, deal, driftwood, fagot, hardwood,
   kindling, kindlings, lath, lathing, lathwork, log, lumber,
   panelboard, paneling, panelwork, plank, planking, plyboard,
   plywood, pole, post, puncheon, shake, sheathing, sheathing board,
   sheeting, shingle, sideboard, siding, slab, slat, softwood, splat,
   stave, stick, stick of wood, stovewood, three-by-four, timber,
   timbering, timberwork, two-by-four, weatherboard, wood, yule clog,
   yule log