The network firewall represents the first line of defense against Man-in-the-Middle (MitM) attacks, which threaten the confidentiality, integrity, and authenticity of digital communications. This paper offers a systematic classification of core MitM techniques—ranging from ARP poisoning and DNS spoofing to HTTPS degradation (SSL stripping) and session hijacking—alongside specialized variants targeting cloud services, browsers, mobile applications, and IoT devices. Particular attention is given to vulnerabilities in VPN infrastructure, where centralized traffic decryption creates high-value targets, as well as weaknesses in IoT ecosystems due to unvalidated certificates and outdated factory settings. An analytical-comparative methodology is applied, encompassing a literature review, statistical assessment of the economic impact of MitM incidents, and a practical demonstration of advanced firewall capabilities via Linux iptables/nftables configuration. The paper details both fundamental and advanced features of modern firewall solutions, including ACL rules, stateful inspection, application-layer filtering, DNS filtering, TLS inspection, and integration with IDS/IPS systems. Illustrative examples from popular application environments highlight the strengths and limitations of these measures. The findings emphasize that while the firewall is essential, it is not sufficient on its own. Effective defense requires a multilayered architecture that combines encrypted DNS requests, strict TLS certificate validation, anomaly detection, and continuous user education to significantly reduce the risks and economic consequences of MitM attacks in contemporary digital networks.