Examine source code of Element Spacing Inspector

{
  "manifest_version": 2,
  "name": "Element Spacing Inspector",
  "version": "1.0",
  "description": "Inspect the spacing (margin and padding) of any element on a webpage by hovering over it.",
  "icons": {
    "48": "icon.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "icon.png",
    "default_popup": "popup.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{6525f692-ceab-4b8a-b1ca-922dda12c638}"
    }
  }
}