The Collaborative International Dictionary of English v.0.48:

Firing \Fir"ing\, n.
   1. The act of discharging firearms.
      [1913 Webster]

   2. The mode of introducing fuel into the furnace and working
      it. --Knight.
      [1913 Webster]

   3. The application of fire, or of a cautery. --Dunglison.
      [1913 Webster]

   4. The process of partly vitrifying pottery by exposing it to
      intense heat in a kiln.
      [1913 Webster]

   5. Fuel; firewood or coal. [Obs.] --Mortimer.
      [1913 Webster]

   Firing iron, an instrument used in cauterizing.
      [1913 Webster]

WordNet (r) 3.0 (2006):

firing
    n 1: the act of firing weapons or artillery at an enemy; "hold
         your fire until you can see the whites of their eyes";
         "they retreated in the face of withering enemy fire" [syn:
         fire, firing]
    2: the act of discharging a gun [syn: discharge, firing,
       firing off]
    3: the act of setting something on fire [syn: ignition,
       firing, lighting, kindling, inflammation]
    4: the termination of someone's employment (leaving them free to
       depart) [syn: dismissal, dismission, discharge,
       firing, liberation, release, sack, sacking]

Moby Thesaurus II by Grady Ward, 1.0:

175 Moby Thesaurus words for "firing":
   afflatus, agitation, alcohol, animating spirit, animation, animus,
   antiaircraft fire, archery, arousal, benzine, briquette, burnable,
   butane, carbon, cashiering, casting, charcoal, chucking, coal,
   coke, combustible, conge, cross fire, curtain fire, deconsecration,
   defrocking, deposal, deposition, deprivation, dethronement,
   direct fire, disbarment, disbarring, discharge, discrownment,
   disemployment, disenthronement, dismissal, displacement,
   displacing, divine afflatus, dope, drumming out, dry fire,
   enlivenment, ethane, ethanol, exasperation, excitation, excitement,
   excommunication, exhilaration, expulsion, file fire, fire,
   fire of demolition, fireball, firepower, fireworks, flack, flak,
   flammable, flammable material, flammation, flinging, fomentation,
   forced resignation, forced separation, fuel, fuel additive,
   fuel dope, furloughing, gas, gas carbon, gasoline, genius,
   ground fire, gunfight, gunfire, gunnery, gunplay, heaving, heptane,
   hexane, high-angle fire, horizontal fire, hurling, ignition,
   impeachment, incitation, incitement, infection, inflammable,
   inflammable material, inflammation, infusion, inspiration,
   instigation, interdiction fire, irritation, isooctane, jaculation,
   jet fuel, kerosene, kicking upstairs, kindling, layoff, lighting,
   lighting up, liquidation, lobbing, machine-gun fire, methane,
   methanol, mortar fire, moving spirit, musketry, natural gas,
   octane, oil, ousting, overthrow, overthrowal, paraffin, peat,
   pensioning off, pentane, pep rally, pep talk, percussion fire,
   pink slip, pistol fire, pitching, projection, propane, propellant,
   provocation, purge, rabble-rousing, raking fire, rapid fire,
   removal, retirement, ricochet fire, rifle fire, rocket fuel,
   shellfire, shoot-out, shooting, skeet, skeet shooting, slinging,
   stimulation, stirring, stirring-up, superannuation, surplusing,
   suspension, the ax, the boot, the bounce, the gate, the sack,
   throwing, ticket, time fire, trajection, trapshooting, turf,
   unchurching, unfrocking, unseating, vertical fire, walking papers,
   zone fire