As of Patch 3.0.2 Firestones and Spellstones now apply a temporary effect to a Warlock's weapon. The Spellstone improves the damage done by periodic effects and grants spell haste. The Firestone improves the damage done by direct effects and grants increased critical strike chance.
Firestones and Spellstones were previously equipped in the wand slot and before that in the off-hand slot. The Spellstone gave the user an improved critical strike chance and could be used to dispel magical debuffs. The Firestone gave bonus fire damage to the weapon of the user (only for melee strikes) and also granted a damage bonus to all fire spells cast by the warlock.