Sigh. Looking at this and the walk-through you would think Mozilla would have a little more vision. SQL queries on the client? Guess I was secretly hoping Mozilla would adopt something more along the lines of CouchDB.
Wow. Sorry serious case of not reading the code closely enough because of work. You're totally right. In fact they mention how easy it would be to write a CouchDB layer on top! Sheesh.
They show two different client side storage mechanisms. WebDB and IndexedDB. The web one is SQLite, it exists right now on most of the modern browsers. IndexedDB is what they're planning on implementing, if you read the article, it's essentially the basis for something like CouchDB.