Stress is a major factor for the degradation of cardiac health in first responder professionals such as firefighters. Monitoring stress during real events might be the key for controlling this problem. In this paper we inspect how standard heart rate variability (HRV) measures are associated with the self-perception of stress of firefighters in action, supported by an advanced technological solution to acquire this data. Results obtained from more than 94 hours of annotated ECG recordings of firefighters in action are promising, showing positive association with various standard HRV measures. Given the richness of the gathered data, we have also measured the association of the HRV measures with the stage of a firefighting event (pre, during, post), obtaining some interesting results that hint that the psychological impact of the post-event may be one of the most concerning situations for a firefighter, motivating further studies on this in the future.