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Background: Firefighting is a hazardous occupation, yet data online-of-duty deaths in European firefighter populations remain limited. This study aimed to characterise the mechanisms and circumstances of firefighter fatalities in Poland and to estimate exposure-based fatality rates, with particular attention to differences between career and volunteer personnel. Methods: In this retrospective observational study, line-of-duty firefighter fatalities in Poland from 1995 to 2025 were identified from a nationwide repository and cross-validated against official sources. The mechanism of death was classified from case narratives following the Utstein framework. Group comparisons used chi-square, Fisher’s exact, and Welch’s tests; multivariable probit regression assessed predictors of mechanism; and per-capita and per-deployment fatality rates were computed using national denominator data. Results: Of 112 fatalities, 73 (65.2%) involved volunteer firefighters. Sudden Cardiac Arrest of Presumed Non-Traumatic origin (SCA-PNT) was the leading mechanism (44.6%), followed by traumatic injury (37.5%). Volunteers were older than career firefighters (46.4 ± 14.0 vs. 34.6 ± 8.7 years; p < 0.001) and more likely to die of SCA-PNT (odds ratio 6.35; 95% confidence interval 2.46–16.40) and during the response phase (odds ratio 5.07; 1.89–13.55). Age was the strongest independent predictor of mechanism. The per-capita fatality rate was higher among career firefighters (incidence rate ratio 5.16), whereas the per-deployment rate was higher among volunteers (incidence rate ratio 2.25). Conclusions: Firefighter mortality in Poland differs by employment status and is strongly age-dependent. Age-stratified cardiovascular surveillance and prevention may be more effective than formation-based approaches.