The Collaborative International Dictionary of English v.0.48:

firedamp \firedamp\ n.
   a mixture of gases (mostly methane) that forms in coal mines
   and becomes explosive when mixed with air. It is a source of
   serious hazard in coal mining operations.
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

firedamp
    n 1: a mixture of gases (mostly methane) that form in coal mines
         and become explosive when mixed with air

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "firedamp":
   afterdamp, blackdamp, breath, chokedamp, cloud, coal gas, damp,
   effluvium, exhalation, exhaust, exhaust gas, fetid air, flatus,
   fluid, fume, malaria, mephitis, miasma, puff of smoke, reek, smoke,
   smudge, steam, vapor, volatile, water vapor