The modernization of the electric grid, utilizing advances in information technologies, has uncovered opportunities for improved communication and efficiency of power systems. While this smart grid environment offers significant economic and reliability benefits, it has also divulged a substantial range of cyber attack issues. Attacks on embedded microprocessor-based devices, like Stuxnet, have demonstrated that the firmware of control equipment can be susceptible to security breaches able to cause great damage to the system. Despite the numerous studies on power systems security, researchers do not have a representative set of benchmarks and taxonomy classes to be able to develop proper detection and mitigation strategies against such attacks. To this end, we provide a taxonomy for firmware Trojans and develop firmware Trojan benchmark cases for four devices commonly found in smart grid deployments.