Examine source code of Steam background preview

{
  "manifest_version": 2,
  "name": "Steam background preview",
  "version": "1.1.2",
  "homepage_url": "https://github.com/Ratott/profile-preview",
  "description": "Steam profile background preview extension, adds a bar to the top bar of the steam page to change the background of your profile easier and quicker.",
  "icons": {
    "48": "icon-48.png",
    "96": "icon-96.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.steamcommunity.com/id/*",
        "*://*.steamcommunity.com/profiles/*"
      ],
      "js": [
        "preview.js"
      ]
    }
  ],
  "browser_action": {
    "default_popup": "popup.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{ca97e167-bc87-4133-81f0-068c9ec63ce3}"
    }
  }
}