They're using the Web SQL database to store the books on the client. This has never been supported by Firefox. Alternatives are becoming available, but it's not surprising that they started with the most established client-side database. It's also the only one supported by Safari, and since it seems like tablet support is one of their goals, that's important.
They probably will. They don't need to support everything off-the-bat, particularly since it doesn't seem like desktop browsers are even the primary target.
