CloudBoost: CloudBoost.io is the best alternative to Firebase.
With a plethora of great features such as search, cache,queue, push notifications, daily backup,offline sync,relation with docker support, slack integration for notifications in case of any event and the ability to push any file/image/music to it, CloudBoost gives us all we could ever wish for.
Not just this its Open Source too giving us the ability to use it the way we love to without worrying about vendor lock-in.
With CloudBoost app development is a breeze. You can checkout / contribute to open source offering here : CloudBoost/cloudboost.