Examine source code of Pixel measurement

{
  "version": "2.1.0",
  "name": "Pixel measurement",
  "description": "Pixel measurement Firefox extension — Measure lengths in pixels directly in your browser.",
  "default_locale": "en",
  "manifest_version": 2,
  "author": "Elena Semenova",
  "offline_enabled": true,
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "permissions": [
    "storage",
    "activeTab"
  ],
  "browser_action": {
    "default_title": "Pixel Measurement",
    "default_icon": {
      "16": "./icons/OFF/16x16.png",
      "48": "./icons/OFF/48x48.png",
      "64": "./icons/OFF/64x64.png",
      "128": "./icons/OFF/128x128.png"
    }
  },
  "icons": {
    "16": "icons/16x16.png",
    "48": "icons/48x48.png",
    "64": "icons/64x64.png",
    "128": "icons/128x128.png"
  },
  "key": "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAgRp++OXEjBCePwKS5/GjVAWoNMjulFz//rb8Qa6ARAS7RIAlA61hxTJQqHpt+IjQ6oS1RGbbNbmWVbT8KINTTr7aRLe1o+rnjYdSF+IB5XHXLW7Sm860rm3GjZVXJjEOcvfSsVoWL5PdeKcIQKZ6Zg3waHZNzb27GEvqXL/sT328sDSBjvaIkkxjqLbaIwixZ7pyOpHCnXWgu3ieaPgmby8/PUtIolHPyphRObr+JybJGpUjJIPYiYVySEXcGZr35HCEIs9+frOzRVA2O2ypxXhfIkcM/o+H0yGzWGNi7pjvmiumVu4Vn900dKI8jYS5Uk+VzWrHVd3ukHUTT5CK2QIDAQAB",
  "externally_connectable": {
    "matches": [
      "https://onlineapp.pro/*",
      "https://onlineapp.stream/*",
      "https://onlineapp.live/*"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{25ff1532-8ff9-461c-8933-0abaa694073c}"
    }
  }
}