How to edit an IndexedDB file?

I'd like to edit an IndexedDB outside of my browser (Firefox), since the dev tools do not offer an editing feature (which is btw really unhandy, imo).

I located the SQLite file and opened it with different tools, but everytime I try to delete a value, I get an error like this:

no such function: update_refcount: delete from object_data where object_store_id = 7;

This leaves me with the conclusion that the format used for the db is non-standard SQLite. Is that correct? How to edit it anyway?