The Collaborative International Dictionary of English v.0.48:

firebug \firebug\ n.
   a criminal who illegaly sets fire to property; an arsonist.

   Syn: arsonist, incendiary.
        [WordNet 1.5]

   2. a brightly colored type of true bug that can exude a
      stain.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

firebug
    n 1: a criminal who illegally sets fire to property [syn:
         arsonist, incendiary, firebug]
    2: a true bug: brightly colored bug that can exude a stain