The Collaborative International Dictionary of English v.0.48:

Firebrand \Fire"brand`\, n.
   1. A piece of burning wood. --L'Estrange.
      [1913 Webster]

   2. One who inflames factions, or causes contention and
      mischief; an incendiary. --Bacon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

firebrand
    n 1: a piece of wood that has been burned or is burning [syn:
         brand, firebrand]
    2: someone who deliberately foments trouble; "she was the
       instigator of their quarrel" [syn: instigator, provoker,
       inciter, instigant, firebrand]

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "firebrand":
   Mafioso, Young Turk, agent provocateur, agitator, agitprop, beast,
   beldam, berserk, berserker, bomber, brand, brute, burning ember,
   butane lighter, catalyst, cigarette lighter, cinder, coal,
   demagogue, demon, devil, dragon, ember, exciter, fiend, fire-eater,
   flambeau, flint, flint and steel, fomenter, fury, goon, gorilla,
   gunsel, hardnose, hell-raiser, hellcat, hellhound, hellion,
   holy terror, hood, hoodlum, hothead, hotspur, igniter, incendiary,
   inciter, inflamer, instigator, killer, light, lighter, live coal,
   mad dog, madcap, mischief-maker, monster, mugger, portfire,
   provocateur, provoker, rabble-rouser, rapist, revolutionary,
   ringleader, rouser, savage, seditionary, seditionist, she-wolf,
   sparker, spill, spitfire, taper, termagant, terror, terrorist,
   tiger, tigress, torch, tough, tough guy, troublemaker,
   ugly customer, urger, violent, virago, vixen, wild beast, witch,
   wolf



Easton's 1897 Bible Dictionary:

Firebrand
   Isa. 7:4, Amos 4:11, Zech. 3:2, denotes the burnt end of a stick
   (Heb. 'ud); in Judg. 15:4, a lamp or torch, a flambeau (Heb.
   lappid); in Prov. 26:18 (comp. Eph. 6:16), burning darts or
   arrows (Heb. zikkim).