Disclosure: I work on Google Cloud (but disclaimer: not on Firebase).
[1] https://cloud.google.com/images/security/whitepaper-transit-...
Data in Datastore (and Firestore fwiw, that needs an update) is encrypted at rest, and today's paper discusses how it's also encrypted in transit between your app and the service. I don't believe Datastore supports Customer Supplied Encryption Keys [2] (yet) but that lets you have a key that even Google can't decrypt the data without you providing it. That's a pretty good way to separate permission from keys.
Disclosure: I work on Google Cloud.
[1] https://cloud.google.com/security/encryption-at-rest/default... [2] https://cloud.google.com/security/encryption-at-rest/custome...
Firebase hosting also serves content over HTTPS and redirect all http requests to https.