Firefighter uniforms provide thermal protection and block radiant as well as high-temperature heat. However, they limit cooling and the dissipation of internal heat from the body. This study attempted to improve firefighter uniforms by developing and evaluating a comfortable and thermally balanced design. This study recruited six male college students for a within-subject comparison of vests with and without active liquid cooling. The participants used a questionnaire to report their comfort level in each body part while performing various motions. A biomechanical analysis was performed to objectively evaluate the comfort level of the cooling vest before and after the test. Subsequently, the participants’ blood pressure and ear as well as skin temperature were measured as they ran on a treadmill. The participants also responded to a questionnaire regarding their thermal perception. The results revealed that the cooling vest was comfortable and flexible. The data from the psychological questionnaire indicated that the participants were satisfied with the warmth, coolness, and other various aspects of the cooling vest. Moreover, the cooling vest positively affected the wearer’s microclimate and provided a comfortable thermal balance. The current findings demonstrate the feasibility of using human-factors-engineering-based objective verification methods for designing products.