Examine source code of Measure Overlay

{
  "manifest_version": 2,
  "name": "Measure Overlay",
  "description": "Drop a measuring overlay onto the current tab: precise cursor position readout and rectangle dimensions for layout work.",
  "version": "5.0",
  "browser_specific_settings": {
    "gecko": {
      "id": "{5539713e-8f6c-4cdf-8745-27bb19a32460}",
      "strict_min_version": "101.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "icons": {
    "32": "images/icon-32.png",
    "48": "images/icon-48.png",
    "64": "images/icon-64.png",
    "96": "images/icon-96.png",
    "128": "images/icon-128.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_title": "Measure Overlay",
    "default_popup": "scripts/popup.html",
    "default_icon": {
      "32": "images/icon-32.png",
      "48": "images/icon-48.png",
      "64": "images/icon-64.png",
      "96": "images/icon-96.png",
      "128": "images/icon-128.png"
    }
  }
}