I currently use WebSQL for offline capabilities in my app. Just have no interest at all in learning IndexedDB, especially when WebSQL is still supported and when its similar enough to other SQL technologies that I don't have to spend to much time learning it.
Google uses it for all offline apps (Gmail, Docs, Calendar).
Amazon uses it for the Kindle HTML5 app.
The best thing to do is to use WebSQL. That way, Mozilla will be forced to implement it or become less useful than other browsers.
It's the only way. IndexedDB is a clusterf*ck. Poor performance, hand-rolled joins, etc, etc.