The next Firefox 4 beta is set to include support for the IndexedDB standard, which isn't actually a standard yet.
IndexedDB provides a database for storing data that may be useful even when the user is offline.
"IndexedDB allows Web apps to store large amounts of data on your local system (with your explicit permission, of course) for fast offline retrieval at a later time," Ben Turner the developer of IndexedDB for Firefox said. "We're hoping that Web mail, TV listings, and online purchase history will one day be as convenient to access offline as they are online."
IndexedDB is being included in nightly Firefox builds and the ninth Firefox 4 beta. Firefox 4 is expected to be released in February.