From the How It Works page: "Developers include a JavaScript library in their application which gives them access to a shared data structure (your Firebase). Any changes made to that data structure are automatically propagated to all other clients and to the Firebase servers....If a user modifies a piece of data a callback will be triggered on every client allowing it to respond to the change."

It sounds like every client gets a full copy of the entire database. You wouldn't exactly want to run reddit on this.

Or am I missing something?