Examine source code of Layout Ruler

{
  "manifest_version": 2,
  "name": "Layout Ruler",
  "version": "1.0",
  "description": "Measure elements on a webpage to get pixel-perfect dimensions and positioning.",
  "icons": {
    "48": "icon.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "icon.png",
    "default_popup": "popup.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{358eddba-b0ac-47f0-8ba2-09c045856f0f}"
    }
  }
}