The new Firefox OS will be built around Gecko, the engine behind Firefox.
According to the article, 75% of “apps” (I hate that term) are already written in HTML5, which is entirely likely. Given the cross platform nature of HTML5 many of those “instant iOS” apps will have no problem migrating to the Firefox OS.
From the article:
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.