Examine source code of Performance Map

{
  "name": "Performance Map",
  "description": "Heatmap of resources loaded in the browser using the Resource Timing API",
  "version": "1.2.0",
  "permissions": [
    "tabs",
    "activeTab",
    "http://*/*",
    "https://*/*"
  ],
  "browser_action": {
    "default_icon": "icons/logo-16.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "applications": {
    "gecko": {
      "id": "@perfmap",
      "strict_min_version": "57.0"
    }
  },
  "icons": {
    "16": "icons/logo-16.png",
    "32": "icons/logo-32.png",
    "128": "icons/logo-128.png"
  },
  "manifest_version": 2
}