Let us dive deep into Gaia. But wait! Before getting started, let us look into the architecture.
Image Source: MDN, licensed under CC-BY-SA 2.5.
Bewildered? Firefox OS is of three-layered architecture. The base being the low level linux kernel (i.e.,) Gonk and above that lies Gecko, a layout engine which reads HTML, CSS, JS, XUL, etc and render it. And at the top level, lies Gaia which is the User-Interface for Firefox OS.