From this diagram I understand that the last step of the bootup process has the window manager launching the Gaia home screen.
I want to experiment and change the the first app that is booted and whatever launches when the home button is pressed, but I have not been able to figure out exactly where is that the homescreen app first launched.
Any pointers would be greatly appreciated.
There is a setting called homescreen.manifestURL that points to the manifest URL of the application that is used as homescreen. You can grep this string to know where exactly it's used.
This setting is set at build time in build/settings.js. You can try to change it there.
You can also install a separate homescreen app (see https://github.com/KevinGrandon/firefoxos-homescreen-boilerplate for a boilerplate for a homescreen -- I'm not sure how uptodate it is) and enable it from the Settings app. It has a "role": "homescreen" property in its manifest.
