Offline cache support was added to the Firefox trunk last night. Check out bug 367447 for details. This is a major step in Firefox 3′s offline webapp support. There is still some more things to add, however, including an API for web developers. I’d expect the API to “feel” like some of the WHATWG APIs for web applications. I’d also expect drafts of any APIs to be ready for review soon. In fact, bug 371432 does have some API discussion.