The answer is: traditional authority.
According to Max Weber, there are three types of authority that exist in societies and that legitimize different forms of domination: traditional authority; charismatic authority, and legal or rational authority.
Traditional authority is legitimized by traditional rights that have been granted to certain people or groups, by commonly accepted customs. It is maintained and reproduced by the culture and it is not likely to be questioned.
In this example, people tend to obey firefighters, based on an accepted custom, because they believe that firefighters withhold a right to exercise authority. This authority is not legitimized by laws or regulations, which would be the case of a rational authority, but by a belief that firefighters are capable of managing emergency situations and therefore, should be obeyed.