Mozilla is working on a new mobile operating system to compete with iOS, Android, and Windows Phone 8, among others. The new OS is developed using HTML5 as the basis and reportedly uses quite a bit less power than its competitors. It would appear that devices have already been distributed to developers.
The device that has reportedly been distributed is a device based upon ZTE's V788D Android smartphone. The only change is the important part: the OS. The specifications of the device include a 3.5-inch 800x480 screen, 512MB RAM, 5 megapixel camera, and a 1.5GHz SoC. Not a high spec phone by any comparison.
However, by taking an existing phone and changing the operating system, Mozilla is able to produce a device with the new mobile operating system quickly which allows it to get into the hands of developers sooner. This allows the OS to be developed quickly so that it isn't left completely behind by iOS and Android.