Examine source code of Fire-Track

{
  "manifest_version": 2,
  "name": "Mail Tracker for Webmail",
  "version": "1.0",
  "description": "Tracks if emails have been opened by injecting a tracking pixel.",
  "icons": {
    "48": "icons/icon.png"
  },
  "permissions": [
    "tabs",
    "<all_urls>"
  ],
  "content_scripts": [
    {
      "matches": [
        "*://mail.google.com/*"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "browser_action": {
    "default_icon": "icons/icon.png",
    "default_title": "Mail Tracker"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{f021a775-d658-41ec-b620-dc21bea3f205}"
    }
  }
}