Firefighters face stressful life-threatening events requiring fast decision-making. To better prepare for those situations, training is paramount, but errors in real-life training can be harmful. Virtual reality (VR) simulations provide the desired realism while enabling practice in a secure and controlled environment. Firefighters' affective states are also crucial as they are a higher-risk group. To assess the impact on affective states of 2 simulated immersive experiences in a sample of healthy firefighters (before, during, and after the simulation), we pursued a multivariate approach comprising cognitive performance, situational awareness, depression, anxiety, stress, number of previous adverse events experienced, posttraumatic stress disorder (PTSD) severity, and emotions. The efficacy and ecological validity of an innovative VR haptic system were also tested, exploring its impact on performance. In collaboration with the Portuguese National Fire Service School, we exposed 22 healthy firefighters to 2 immersive scenarios using the FLAIM Trainer VR system (neutral and arousing scenarios) while recording physiological data in a quasi-experimental study. Baseline cognitive performance, depression, anxiety, stress, number of adverse events, and severity of PTSD symptoms were evaluated. Positive and negative affective states were measured before, between, and after each scenario. Situational awareness, sense of presence, ecological validity, engagement, and negative effects resulting from VR immersion were tested. Baseline positive affect score was high (mean 32.4, SD 7.2) and increased after the VR tasks (partial η2=0.52; Greenhouse-Geisser F1.82,32.78=19.73; P<.001). Contrarily, mean negative affect score remained low (range 11.0-11.9) throughout the study (partial η2=0.02; Greenhouse-Geisser F2.13,38.4=0.39; P=.69). Participants' feedback on the VR sense of presence was also positive, reporting a high sense of physical space (mean score 3.9, SD 0.8), ecological validity (mean score 3.8, SD 0.6), and engagement (mean score 3.8, SD 0.6). Engagement was related to the number of previously experienced adverse events (r=0.49; P=.02) and positive affect (after the last VR task; r=0.55; P=.02). Conversely, participants reported few negative effects (mean score 1.7, SD 0.6). The negative effects correlated positively with negative affect (after the last VR task; r=0.53; P=.03); and avoidance (r=0.73; P<.001), a PTSD symptom, controlling for relevant baseline variables. Performance related to situational awareness was positive (mean 46.4, SD 34.5), although no relation was found to metacognitively perceived situational awareness (r=-0.12; P=.59). We show that VR is an effective alternative to in-person training as it was considered ecologically valid and engaging while promoting positive emotions, with few negative repercussions. This corroborates the use of VR to test firefighters' performance and situational awareness. Further research is needed to ascertain that firefighters with PTSD symptomatology are not negatively affected by VR. This study favors the use of VR training and provides new insights on its emotional and cognitive impact on the trainee.