The Collaborative International Dictionary of English v.0.48:

arsonist \arsonist\ n.
   1. 1 a criminal who illegaly sets fire to property.

   Syn: incendiary, firebug.
        [WordNet 1.5]

   Note: The definition of this crime is varied by statues in
         different countries and states. The English law of
         arson has been considerably modified in the United
         States; in some of the States it has been materially
         enlarged, while in others, various degrees of arson
         have been established, with corresponding punishment.
         --Burrill.
         [1913 Webster]

WordNet (r) 3.0 (2006):

arsonist
    n 1: a criminal who illegally sets fire to property [syn:
         arsonist, incendiary, firebug]