Can 发火 and 着火 be used interchangeably?
发火 is usually used metaphorically for a person, meaning getting angry. For example, you can say to someone 别发火/don't get angry, 他又发火了/he is angry again. 

着火 is usually used as its literal meaning for a real object. Xxx 着火 means xxx catching fire/be on fire. For example, 房子着火了/ the house caught fire. 

Basically, they won't be used interchangeably. We don't say 他又着火了 or 房子发火了.