Apps are of not much use if they don't work offline. That's partly why we prefer them to just opening a browser and looking for content on our phones. Our users will be offline - it just happens - and we need to ensure that our applications are still usable then. This is especially important as the term "Web applications" sounds like you need to be connected to use them. In many cases we are on a plane, underground or used up our data allowance on the SIM we are using, which is why it is important that apps built for Firefox OS work offline. HTML5 offers a few technologies that allow for offline functionality, mainly AppCache and DOMStorage.