The aim of the study was to evaluate the association of the quality of diet as calculated by the Nutrient Rich Food index (NRF9.3), and length of service (LS) (≤10 years vs. >10 years) with selected serum biochemical parameters, the proportions of different lipid profile fractions and advanced glycation endproduct (AGE) values of 108 firefighters from the State Fire Service in Wroclaw. The LS officers > 10 years had significantly higher total cholesterol (211.50 (184.00–254.00) vs. 184.00 (166.00–194.00)), LDL (123.75 (108.20–167.90) vs. 105.18 (90.24–119.00)) non-HDL (151.70 (132.00–196.70) vs. 122.00 (106.00–140.00)), triglycerides (118.50 (96.00–158.00) vs. 78.00 (67.00–103.00)) and lower HDL concentrations (51.30 (45.60–56.70) vs. 58.00 (51.70–66.10)) compared to firefighters in the LS ≤ 10 years group. Significant differences between the seniority groups were also noted for all lipid profile ratios. Regardless of the officers’ seniority, systolic blood pressure was observed at the highest normal level of 134.4 ± 14.4 in the LS ≤ 10 years group and 139.5 ± 14.3 in the LS > 10 years group. Advanced glycation endproduct values were significantly dependent on diet quality, as expressed by the NRF9.3 index and on the TG/HDL ratio, but not on seniority. Diet quality, as expressed by the NRF9.3 index, had a significant association with GLU and FI levels, and components of the lipid profile between seniority groups. As NRF9.3 increased, TG/HDL, LDL/HDL, TC/HDL, and non-HDL/HDL ratios decreased. AGEs were significantly affected by NRF9.3 and significantly associated with TG/HDL. Firefighters’ diets, as assessed by the NRF9.3 index, had a significant association with predictors of insulin resistance, diabetes, and cardiometabolic predictors between seniority groups. The nutritional education of firefighters (and other professional groups working irregularly), especially those with longer tenure (e.g., >10 years), is necessary to prevent the development of, e.g., CVD, MetS, and T2DM, which contribute towards a reduced ability to perform professional duties.