Is there support for old Firebird databases as a custom user database?
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The custom databases will allow to connect to any user store that is accessible through the Internet; the custom scripts you configure are basically Node.js code with some constraints on the packages that can be used. However, you can put an HTTP API in front of your user store and call that API from the scripts so technically you can use pretty much anything as the underlying storage.
See these docs for a way to search which NPM modules can be used from custom databases: