Spanish wireless network Telefónica recently started selling a super-cheap Linux-driven phone called the ZTE Open for the equivalent of around $90, which includes about $40 worth of prepay. Essentially, it's a $50 smartphone. Aside from the bottom-end retail price, what's unique about this smartphone is that it's the first to use a new mobile operating system called Firefox OS. Like Android, Firefox OS is based on Linux. A highly adaptable markup language called HTML5, however, is its biggest differentiating feature.