Mozilla had announced plans to create its own mobile OS based on HTML5 technologies. Some of the builds of this OS are available but it's yet to release on any devices. However, Oleg Romashin has successfully ported Firefox OS to run on Raspberry Pi and builds are available for download.
The below video shows Firefox OS running in Raspberry Pi.
Raspberry Pi is a credit card sized single board computer that costs as low as $25. A range of operating systems including Debian, Fedora, Kubuntu, Android and Arch Linux have been successfully ported to run on it.