The Mozilla Foundation has just renamed the project Boot to Gecko “Firefox OS”. But can we really talk about an operating system?
Absolutely. In terms of architecture, it is an operating system based on Linux, just as Android is. But we rely on Gecko, the Firefox web browser layout engine, to run applications written entirely in HTML5. We dropped XUL (the XML User Interface Language) in favour of HTML5, a language known to all web developers.
Even native applications, such as the dialer or address book, are written in HTML5, and users will be able to examine the source code to check it.
via Firefox OS Will Get Overwhelming Developer Support – Mozilla.