Examine source code of Flexbox Detector

{
  "manifest_version": 2,
  "name": "Flexbox Detector",
  "description": "Detects whether a site uses Flexbox",
  "version": "1.1.1resigned1",
  "icons": {
    "16": "icon/16.png",
    "32": "icon/32.png",
    "48": "icon/48.png",
    "64": "icon/64.png",
    "128": "icon/128.png"
  },
  "browser_action": {
    "default_title": "Check if this site uses Flexbox",
    "default_icon": "icon/32.png",
    "default_popup": "popup/check_flexbox.html"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{487e3825-c7ad-4439-aadd-02525519b0b5}"
    }
  }
}