This is the first study investigating the determinants of mental health among Japanese firefighters with the COVID-19 outbreak. Firefighters who respond to emergency calls and administer emergency medical treatment face numerous serious threats to their mental health because of the spread of COVID-19. These threats range from significant infection risks to changes in daily operations. We conducted an online questionnaire survey with Japanese firefighters (n = 2,748, valid responses = 2,204) who also served as ambulance workers during the second COVID-19 wave in August 2020. The questionnaires inquired about personal and occupational demographics, occupational stress resulting from COVID-19, social support, germ aversion using a subscale of the Perceived Vulnerability to Disease (PVD) scale, and the General Health Questionnaire (GHQ-12). The results indicated that the burdens of infection prevention, anxiety about infection and furloughs, interpersonal conflicts related to COVID-19, reduced social support, and germ aversion were associated with deterioration of mental health. Measures for maintaining the mental health of firefighters are discussed based on these results.