Firefox OS is an open source mobile operating system developed by Mozilla. It uses open web technologies like HTML5, CSS and JavaScript to build apps, allowing developers to build once and deploy apps across devices without needing to learn different native platforms. The OS consists of Gecko for rendering web content, Gaia for the user interface, Gonk as the Linux-based kernel, and various web APIs for functionality like telephony, SMS, contacts and more. Developers can publish apps to the Mozilla Marketplace or install/host them directly on devices.