Is firebase realtime database MongoDB?

Firebase is a Backend-as-a-Service containing identity management, realtime data views and a document database . It runs in the cloud. MongoDB on the other hand is a full fledged database with a rich query language. In principle it runs on your own machine, but there are cloud providers.

Firebase Realtime Database is a cloud-hosted database that supports iOS, Android, Web, C++ and Unity platforms . Realtime means that any changes in data are reflected immediately across all platforms and devices within milliseconds.

Firebase is a real-time object store. It is not a SQL database and is not intended to be a replacement for one. It completely lacks mechanisms such as JOINs, WHERE query filters, foreign keys, and other tools relational databases all provide.