Examine source code of Mail Tracker for Firefox

{
  "manifest_version": 2,
  "name": "Mail Tracker for Firefox",
  "version": "1.0",
  "description": "Check if your email is opened by the recipient.",
  "permissions": [
    "activeTab",
    "storage",
    "https://mail-tracker-production-7e26.up.railway.app/*"
  ],
  "icons": {
    "48": "icon.png"
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icon.png"
  },
  "content_scripts": [
    {
      "matches": [
        "https://mail.google.com/*"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_end"
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "amardeepkumar13641364@gmail.com"
    }
  }
}