Since the late 80s, firewalls are at the heart of network security. First designed to enable private networks to be opened up to the outside in a secure way, the growing complexity of organizations make them indispensable to control information flow within a company. The central role of firewalls in the security of the organization information make their management a critical task. That is why for years many works have focused on checking and analysing firewalls. In this paper, we propose a new approach for analysing firewalls, based on tree automata techniques: we show that tree automata provide a way to compare firewalls and to perform all usual analysis of firewalls (including the network address translation (NAT) functionality) in a unique formalism.