Examine source code of Link Highlighter is

{
  "manifest_version": 2,
  "name": "Link Highlighter",
  "version": "1.0",
  "description": "Highlight all links on a webpage with one click.",
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": {
      "16": "icon.png"
    },
    "default_title": "Highlight Links",
    "default_popup": "popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{c884ac16-a782-4f17-b7c1-29cb6443cbd5}"
    }
  }
}