Firefox OS is a mobile operating system based on open web technologies like HTML5, CSS and JavaScript. Apps for Firefox OS are actually webpages that have access to device hardware through the browser, allowing them to function like native mobile apps. The manifest file contains additional metadata needed to turn a webpage into an installable Firefox OS app, including app information, developer details, permissions and localization settings. The Firefox OS simulator allows testing and running apps without a physical device by launching a simulated Firefox OS environment from the browser's developer menu.