I wouldn't call FirefoxOS a "thin client" (if that's what you meant) - it has a concept of "packaged apps" in which the app and all its assets are packaged and downloaded to the phone, and loaded from there. The built-in "certified" apps (like Phone, Music, Calendar and so on) don't require loading anything from the network at all, for instance.
You can think of FirefoxOS apps the same way conceptually you'd think of them on Android/iOS, except they are always written in open web technologies (HTML/JS/CSS).
App developers do have the option to provide "hosted" apps to the store (which is basically a regular web page but without browser chrome), however even in this case writing HTML5 apps that work offline is totally doable using existing cross-browser standards.