We explored the electronic, optical, and thermoelectric characteristics of novel Cs2XMoBr6 (X=Na, Li) halide-based double perovskites (HDPs) in their monoclinic phase using the density functional theory (DFT). The full-potential linearized augmented plane wave approach was used in the present work. The material Cs2NaMoBr6 was noticed as half-metallic as indicated by an indirect band gap semiconducting nature in the spin-down channel, and with a metallic nature in the spin-up channel. Similarly, the Cs2LiMoBr6 material also revealed a half-metallic nature with an indirect band gap semiconducting nature in spin-down mode and a metallic behavior in spin-up mode. The density of states analysis verifies the electronic behavior of these compounds. To determine the optical characteristics, the optical parameters such as the dielectric functions, the absorption coefficients, the refractivity, the energy loss function, and the reflectivity, have been described. The thermoelectric characteristics of the compounds along the temperature are calculated. It was confirmed that these materials possess extraordinary characteristics that would make them excellent options for influential electronic, optical, and thermoelectric, device applications.