At least mozbrowser keeps it's state opaque and only way of knowing state is to hook into all events and try to maintain it on your own. Examples are current url, loading, canGoBack / canGoForward, title, icon etc.. It would make a lot more sense to expose all of this state as fields as well so components loaded later in the process (like add ons) could find out what the state of things are as they could have missed some events. It also would make API more ergonomic and declarative. See: https://github.com/benfrancis/webview/issues/7

Firefox OS is not being worked on