Combine these offline capabilities with device APIs that allow web apps to access hardware features like cameras, battery level, microphone, etc and Mozilla could effectively break down the barriers not only between HTML5 and native apps, but between high-end mobile devices and low.
Features include the ability to search and browse by artist, album or track, create playlists and download music from Cloud Player to the device for offline playback.
It can even generate an offline playlist on the device -- something similar in spirit to a feature I've long wanted on the iPod and other players, which is to be able to "switch gears" while listening in shuffle mode to play more tracks from just that artist, the rest of the album, or other songs like the one I've heard.