Objectives: The aim of the research was to analyze the impact of selected factors: years of service, the number of interventions, self-efficacy and stress, on occupational burnout. It was hypothesized that firefighters with more years of service and a bigger number of interventions would be characterized by higher perceived stress and burnout, and that self-efficacy would have an impact on reducing the level of perceived stress and burnout. Material and Methods: The participants were firefighters (N = 576) from 12 Polish voivodeships, aged 20–58 years, with different seniority: up to 3, 4–8, 9–15 or >15 years of service. The following research tools were used: the Link Burnout Questionnaire, the 10-item Perceived Stress Scale, the Generalized Self-Efficacy Scale, and an independent questionnaire to gather additional information. A correlation between particular variables was carried out; the Kruskal-Wallis test was performed together with a post-hoc analysis to examine differences in the severity of individual variables depending on seniority, followed by a path analysis studied together with the identification of direct and indirect impacts. Results: The number of interventions did not affect the severity of experienced stress or any of the aspects of burnout. Work experience directly influenced the level of perceived stress (β = 0.219), disillusion (β = 0.076), and relationship deterioration (β = –0.156). The generalized sense of self-efficacy was found to impact both on reducing the sense of stress (β = –0.418) and on all 4 aspects of professional burnout: psychophysical exhaustion (β = –0.181), relationship deterioration (β = –0.16), the sense of professional inefficacy (β = –0.275) and disillusion (β = –0.143). Conclusions: The results have shown that: 1) the number of interventions does not affect the severity of experienced stress or particular aspects of burnout; 2) years of service increase the severity of experienced stress and occupational burnout; 3) self-efficacy has an impact both on reducing the sense of stress and on all aspects of burnout. Int J Occup Med Environ Health. 2020;33(3):283–97