Examine source code of Fast Image Saver

{
  "manifest_version": 2,
  "name": "Fast Image Saver",
  "version": "1.0",
  "description": "Download any Image by Alt+LeftClick. Additional: Click on notification to open folder of downloaded image",
  "permissions": [
    "downloads",
    "notifications"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{6ebfcff4-3470-43ac-b612-fa34a461de8d}"
    }
  }
}