Firefox OS is an open source mobile operating system developed by Mozilla that uses HTML5 technologies and APIs to allow developers to build apps and customize the user interface. It aims to bring the open web to mobile by allowing apps to be built with web technologies like HTML, CSS, and JavaScript instead of native development. The document outlines the key technologies that Firefox OS uses like HTML5, manifest files, and various device APIs for features like vibration, screen orientation, geolocation, and more. It also describes the different types of apps and security levels as well as Mozilla's vision for future web standards and APIs.