As of the time of this question, Firebase backs up all instances daily. So while keeping your own backups may still be useful, it's not essential.
To create your own backups, you can simply curl the data:
curl https://<instance>.firebaseio.com/.json?format=export
Note that for multiple gigabytes of data, this will slow things down and lock read access for a short period. It would be better in this case to chunk the backups and work with smaller portions. The shallow parameter can help here by providing a list of keys for any given path in Firebase, without having to fetch the data first.
curl https://<instance>.firebaseio.com/.json?shallow=true
As previously mentioned, there are also several GitHub libs available for this, and incremental backups are practical with some creativity and a worker thread on the real-time SDK.
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