This paper presents the design of a proportional-integral-derivative power-system-stabilizer using the firefly algorithm for tuning of stabilizer parameters and washout (reset). The proposed optimization of parameters is carried out with eigenvalue analysis based objective function for two cases (two parametric bounds) to guarantee the stability for the single-machine-infinite-bus system model for a wide range of operating conditions. The system performance with Firefly-Algorithm tuned controller is compared with Bat-Algorithm optimized Conventional-Power-System-Stabilizer controller. The power system robustness is tested on 133 operating conditions to set up the superior performance of FA-PID-PSS over the BA-CPSS. According to the eigenvalue analysis and time response parameters results, it is found that BA-CPSS and FA-PID-PSS (case-I) have the ability to stabilize the system for some operating conditions; but the FA-PID-PSS (case-II) can stabilize the system and can improve settling time and overshoot for all operating conditions.