Examine source code of AlwaysFullscreen

{
  "description": "Automatically makes all Firefox windows fullscreen",
  "manifest_version": 2,
  "name": "AlwaysFullscreen",
  "version": "1.1.0",
  "homepage_url": "https://github.com/lucastavaresa/AlwaysFullscreen",
  "background": {
    "scripts": [
      "startup.js"
    ]
  },
  "icons": {
    "64": "icons/fullscreen_64x64.png",
    "128": "icons/fullscreen_128x128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{2cd79bb4-6459-4467-8c50-368c4fbaacbb}"
    }
  }
}