Examine source code of dblclick-image-download

{
  "manifest_version": 2,
  "name": "dblclick-image-download",
  "version": "1.5resigned1",
  "description": "Downloads images when you double-click on them",
  "icons": {
    "48": "icons/icon-48.png",
    "64": "icons/icon-64.png",
    "128": "icons/icon-128.png"
  },
  "background": {
    "scripts": [
      "background/background.js",
      "background/onFocus.js"
    ]
  },
  "permissions": [
    "downloads",
    "tabs",
    "<all_urls>"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{58a45e8a-658a-4f8d-aa1b-62c91b0d580e}"
    }
  }
}