Examine source code of No More 404

{
  "manifest_version": 2,
  "name": "No More 404",
  "version": "1.2",
  "description": "Redirects 404 errors to helpful pages.",
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "storage",
    "<all_urls>"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "32": "icons/logo_32.png",
      "64": "icons/logo_64.png",
      "128": "icons/logo_128.png"
    }
  },
  "options_page": "options.html",
  "icons": {
    "32": "icons/logo_32.png",
    "64": "icons/logo_64.png",
    "128": "icons/logo_128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{c4805820-b588-40b7-9c5c-47b6886ed932}"
    }
  }
}