Firebase Realtime Database is not completely free . There are certain pricing plans. If you want your app to scale you’ll need to pay for the number of connections, disk usage and network usage.
Read moreIs Firebase database expensive?
Firebase Realtime Database allows for 100 simultaneous connections, 1 GB stored, and 10 GB per month downloaded on the free tier. However, the Google Firebase cost on the paid tier translates to 200,000 per database, $ 5 per GB stored, and $ 1 per GB downloaded , while multiple databases per project are permitted.4 Mar 2021
Read more