Firefox OS is a Linux-based operating system that uses the Gecko rendering engine. It implements HTML5, CSS, and JavaScript APIs to power the user interface and basic apps like the browser, calendar, etc. The OS is entirely based on open standards and open source code. It aims to run smoothly on low-end devices, be affordable, and have long battery life by consuming fewer resources than Android and iOS. Firefox OS also allows users to add any web app as a native app, rather than being restricted to an app marketplace. However, some popular apps like WhatsApp are currently missing from Firefox OS.