Firefox OS is an upcoming open source mobile operating system by Mozilla, maker of the Firefox browser. The Linux-based OS is built using HTML5 and other open web standards, in contrast to the “walled garden” approach of Apple’s iOS and many other competing operating systems. Mozilla recently announced that 18 mobile carriers have committed to supporting Firefox OS. Twitter has also announced a Twitter app for Firefox OS. The first Firefox OS devices are expected to go on sale this summer in Europe, South America, and Mexico.