Firefox OS is a mobile operating system built on web standard technologies.
To get started with Firefox OS, I recommend the following book:
Quick Guide For Firefox OS App Development by Andre Garzia.
“Learn how easy and quick it is to develop applications for Firefox OS, the new mobile operating system by Mozilla. Empowered by this books practical approach you will learn thru examples how to develop apps from the beginning all the way to the distribution in the Firefox Markeplace.”
You can download the book for free, or choose to pay the author a reasonable sum for his work 🙂