Firefox OS is an open source mobile operating system developed by Mozilla. It uses HTML5 and CSS3 to create applications, allowing developers to use the same tools and languages as the desktop web. Firefox OS includes components like Gecko for rendering, Gonk as the Linux kernel, and Gaia for the user interface. It provides various web APIs to allow applications access to device functionality. Firefox OS aims to provide an alternative mobile ecosystem that is not a "walled garden" and makes application development simple through available tools and an open source approach. It has been launched on devices from manufacturers like ZTE, LG, and Alcatel.