A full database supported by most major browsers.
PouchDb / CouchDb
PouchDb is a fully JS compliant offline db that is backed by Indexeddb. It syncs with CouchDB implementations and handles conflicts fairly well. This is the only solution that I saw that handles most offline issues out of the box. However, it does add more complexity as you now have to host a CouchDB server.
An offline-first database that is mostly designed for mobile, but not web.
Has support for offline - not offline first, but can handle network disruptions.
Open source platform similar to Firebase in that it can handle network disruptions, but is not designed for offline-first.