Examine source code of Certificate expiration

{
  "manifest_version": 2,
  "name": "Certificate expiration",
  "version": "3.0",
  "description": "Shows how many days until the certificate for the website expires.",
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "<all_urls>",
    "tabs"
  ],
  "browser_action": {
    "default_icon": "/icons/grey-lock.svg",
    "default_title": "How many days until the certificate expires",
    "default_popup": "/html/popup.html"
  },
  "background": {
    "page": "/html/background.html"
  },
  "icons": {
    "48": "/icons/lock.svg",
    "96": "/icons/lock.svg"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{d68d2e5b-8d6c-4867-b566-f3034b6164f7}"
    }
  }
}