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.
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   3. The application of fire, or of a cautery. --Dunglison.
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   4. The process of partly vitrifying pottery by exposing it to
      intense heat in a kiln.
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   5. Fuel; firewood or coal. [Obs.] --Mortimer.
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   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]