I'm developing an android application that will allow users to find informations about the streets in my city. They will be able to register and save some streets as "favourites".
My question is, how can I use Firebase to store the streets data considering that they won't change overtime?
The Firebase data model is not well suited to storing arrays (or Java List objects). See this blog post explaining the behavior you get. Instead of storing a List, follow the Firebase documentation's approach for storing collections. This will indeed store it as a map, which is the correct approach precisely .