Check out my I/O talk on Friday (Progressive Web Apps on Firebase). While the Firebase Web SDK doesn't support persistent offline just yet, we do have some new ways to get the job done.
https://events.google.com/io2016/schedule?filters=Firebase&s...
https://events.google.com/io2016/schedule?filters=Firebase&s...
https://firebase.google.com/docs/database/web/offline-capabi...
However, if you go offline and then try to refresh the content, none of it will be persistently stored or accessible until you go back online.
I was hoping to use Firebase on a React Native app but I do need persistence. Do you have or plan to make some wrapper of the native iOS/Android SDKs for React Native?