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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
There is some crossover between the BaaS of Firebase and what MongoDB Atlas is offering if you are developing using Atlas Sync and Realm. Even so, there is a whole lot more you can find in terms of tutorials and community support for Firebase so it is hard to know how many of the Mongo claims are just future bugs for your project vs what people are currently doing with Firebase.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.