Examine source code of Message Timer

{
  "name": "Message Timer",
  "version": "2.9.8",
  "description": "Times how long it takes to send a message to a content script and back.",
  "content_scripts": [
    {
      "matches": [
        "http://*/*",
        "https://*/*"
      ],
      "js": [
        "page.js"
      ]
    }
  ],
  "browser_action": {
    "default_title": "Time to current page",
    "default_icon": "clock.png",
    "default_popup": "popup.html"
  },
  "manifest_version": 2,
  "browser_specific_settings": {
    "gecko": {
      "id": "{fac35400-51ce-4209-bb08-252d2883ea99}"
    }
  }
}