I was reading about IndexedDB and saw these libraries that were linked to from the MDN page – some of them look interesting:
- PouchDB
- localForage
- dexie.js
- ZangoDB
- JsStore
- Edit: nanoSQL also works with IndexedDB. (nanoSQL article)
More about IndexedDB, including some more libraries at the bottom:
For browser extensions (it should work with Firefox extensions too):
https://developer.ibm.com/clouddataservices/2016/05/11/chrome-extensions-pouchdb/
A tutorial: