Examine source code of hover-dl

{
  "manifest_version": 2,
  "name": "hover-dl",
  "version": "1.4",
  "description": "Hover over an image >250px, click icon to save. Also dsplays a info box with image file size.",
  "permissions": [
    "downloads"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "styles.css"
      ]
    }
  ],
  "icons": {
    "48": "icon.svg",
    "128": "icon.svg"
  },
  "web_accessible_resources": [
    "icon.svg"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "hover-image-downloader@example.com",
      "strict_min_version": "58.0"
    }
  }
}