Firefox OS is Mozilla's open source mobile operating system. It uses HTML5, CSS, and JavaScript to build apps that have access to device APIs for features like notifications, vibration, battery status, and geolocation. Apps are divided into regular web apps, installed web apps with more access, and certified web apps that are critical to device functions. The document outlines several device APIs available to Firefox OS apps and their usage, as well as pre-installed system apps and how to get started developing for the platform.