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The firebomb is a device that fighters can create given an appropriate recipe. The most common such recipe involves certain glands of a fire beetle. This is a first level concoction, following the potion crafting rules, and thus costs 500 GP of components in addition to the special ingredients. Crafting time is one week.
Firebombs inflict two dice of damage on a direct hit (standard thrown weapon attack), and one die of damage to all within a 10′ radius of the explosion (save versus breath for half damage; this applies to the primary target as well if the initial attack was missed). Further, flammable targets may be ignited. The market price for firebombs is around 1000 GP, assuming there are buyers and sellers.
Alchemists and apothecaries can sometimes craft firebombs as well, but they are usually not as well designed for combat purposes (for example, they may consist of liquid in two flasks that must be combined and thrown).
Fighters are generally the only class that knows how to make incendiaries, but, like all things, if someone really wants to play a hybrid class (like a magic user that can craft bombs), we can make that happen (probably in place of being able to create potions, for example).