A usual strategy in both experimental and theoretical studies on bio- and chemiluminescence is to analyze the fluorescent properties of the bio- and chemiluminescence reaction product. Recent findings in a coelenteramide and Cypridina oxyluciferin model raise a concern on the validity of this procedure, showing that the light emitters in each of these luminescent processes might differ. Here, the thermal decomposition path of the firefly dioxetanone and the light emission states of the Firefly oxyluciferin responsible for the bio-, chemiluminescence, and fluorescence of the molecule are characterized using ab initio quantum chemistry and hybrid quantum chemistry/molecular mechanics methods to determine if the scenario found in the coelenteramide and Cypridina oxyluciferin study does also apply to the Firefly bioluminescent systems. The results point out to a unique emission state in the bio-, chemiluminescence, and fluorescence phenomena of the Firefly oxyluciferin and, therefore, using fluorescence properties of this system is reasonable.