Wearing correctly fitting fire gear is essential for firefighters’ safety, work performance, and wearing satisfaction in the field. However, a self-contained breathing apparatus (SCBA) and fire helmet have no size options with specific descriptions. In addition, a male-centered design practice increases these restrictions when female firefighters use the gear. This study aims to provide an optimal fit of SCBA harnesses and fire helmets for firefighters by applying additional fit-adjustable design features. This study identified the design and fit issues that female and male firefighters experience. Based on the study participants’ empirical feedback, fit-adjustable SCBA harness and fire helmet prototypes were developed and then evaluated by 3D body scanning and surveys. The findings indicated an improvement in the range of motion of firefighters’ upper body and wearing stability when wearing the prototypes compared with conventional SCBA harness and fire helmet designs. Through this study, a possibility for fit optimization design was suggested based on the consideration of firefighters’ anthropometric data by gender.