Examine source code of Hover Links Preview

{
  "manifest_version": 2,
  "name": "Hover Links Preview",
  "version": "1.0",
  "description": "Quickly preview the content of a link by hovering over it, without leaving your current page !",
  "browser_action": {
    "default_popup": "popup/index.html"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "main.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{5c1e875c-5d3b-41d4-84cd-8043c3928961}"
    }
  }
}