In a few words, Firefox OS is an environment in which HTML5-based applications are used (not only a markup language, but also all related technologies, including JavaScript, CSS3, web API ...).
These applications run in a special Gecko browser. You can think of Gecko as Firefox with steroids with web page replacement apps. In some aspects, it lacks some browser features, but it implements many APIs to provide secure access to devices.
Gecko is supported by the actual SO, Gonk. Gonk is a very simple distribution of GNU / Linux, and it supports a hardware abstraction layer or HAL between the device hardware and Gecko. Gonk converts low-level hardware access to C ++ interfaces, and Gecko offers these interfaces to developers as standard JavaScript APIs.
Additional Information at the Mozilla Wiki
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