You can also take a look at the source code of the preinstalled/runtime apps that are present in Firefox OS or Gaia, to be precise. For example, the following is an illustration that shows how to open them:
You can click on the Hello World button (in the same place where Open App used to exist), and this will show you the whole list of Runtime Apps, as shown in the preceding illustration. I clicked on the Camera application and it showed me the source code of its
It's completely okay if you are daunted by the huge file. If you find these runtime applications interesting and want to contribute to them, ...