The Collaborative International Dictionary of English v.0.48:

Firefly luciferin \Fire"fly` lu*cif"er*in\, n. (Biochem.)
   a type of luciferin produced by the firefly Photinus
   pyralis.

   Note: Its structure has been elucidated, and chemically it is
         recognized as

         4,5-Dihydro-2-(6-hydroxy-2-benzothiazol-2-yl)-4-thiazolecarboxylic
         acid, C11H8N3O2S2. It has found use in a very
         sensitive assay for ATP, in which concentrations of ATP
         as low as 10^-11 molar can be detected. --[MI11]
         [PJC]