Examine source code of Word Counter g

{
  "manifest_version": 2,
  "name": "Word Counter",
  "version": "1.0",
  "description": "Displays the word count of selected text.",
  "permissions": [
    "activeTab"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "wordcounter-xyz123@example.com",
      "strict_min_version": "58.0"
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "icon.png",
      "48": "icon.png",
      "128": "icon.png"
    }
  }
}