This paper proposes a novel multi-objective optimization (MOO) approach to cooperative path planning of multiple unmanned aerial vehicles (UAVs). The cooperative multi-UAV path-planning problem is formulated in a MOO framework to determine optimal trajectories from the given source to the destination while avoiding collision with obstacles, teammates and mountains. Firefly algorithm with non-dominated sorting (FANS) algorithm is developed to solve the above-mentioned MOO problem with an aim to facilitate rapid convergence of solutions to the optimal Pareto Front, making it particularly suitable for tackling real-world MOO problems. Experiments undertaken with diverse workspaces demonstrate that the proposed algorithm outperforms its state-of-the-art competitors in the current context.