[I work for Firebase]
Firebase should continue to work well for you even if you don't need the persistence features. We're not aware of any case where our persistence actually makes things harder, and in many cases it actually makes your life a lot easier. For example, you probably want to be able to ask "what is the current position of a truck" without needing to wait for the next time an update is sent.
If you've encountered a situation where persistence is actually a negative for you, we'd love to hear about it. That certainly isn't our intention.
Also - we're not Firebase.io -- we're just Firebase (though we do own the firebase.io domain name).