Examine source code of steam-autoage

{
  "manifest_version": 2,
  "name": "Agecheck",
  "version": "1.1",
  "description": "Sets the year to 1999 and presses next after 1 second, saving you 3 clicks.",
  "icons": {
    "48": "icons/border-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.store.steampowered.com/agecheck/*"
      ],
      "js": [
        "agecheck.js"
      ],
      "run_at": "document_end"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{e6691303-419f-4a63-b2c0-e446a0fc5b69}"
    }
  }
}