FireChat is an app used for communication. The convenience of online communication can help large groups organize. However even without FireChat, protesters of the State of Hong Kong would find another way to organize and communicate. Eliminating FireChat would not dissolve protesters' will and does not specifically fuel their cause.
If in this modern day the term "enemy of the State" still exists it clearly refers to people, not mobile applications. People will always find a way to organise themselves in order to conduct protests against issues that arouse strong feelings. It is easier to organise a protest using social networking apps but the app itself is not an enemy.
The app is not an enemy. This is because no one is using the app to promote or commit violent acts. The government can always make using the app illegal, and then it would become an enemy of the state. Until then, there is nothing wrong with organizing protests in this technological way.
The FireChat App should not be an enemy of the State of Hong Kong because it is helping protesters to organize themselves. The protesters represent a large part of the population of Hong Kong, so if an app is helping them to mobilize, then the app is helping the State.