The jury's still out on the color, branded as it may be...
Mozilla's aspirations to release a mobile operating system have been known about for quite some time, and at least two devices have been confirmed. The ZTE Open was first made available in Spain, though today marks the opportunity for UK and US consumers to get the phone following ZTE's announcement.
Rather unusually, the phone's main distribution line is via eBay. There aren't many devices that can say that. Shipping is free, and the phone itself is £60 (or $80). It's not an expensive handset, and the specifications haven't gotten any more lively since it released in Spain, but it's an all-new OS.
Mozilla's downloadable simulator is also available, if you'd like to try the OS on a computer. While not 'feature-complete', it's a decent glimpse at the OS that can be gotten without spending any money.
How Firefox OS is destined to slot into the bigger picture is unclear; it may be aimed solely at budget smartphones, but it may do quite well. In a few years we can ask these questions, much like 'What happened to Windows Phone?'