Examine source code of Page Style Monitor

{
  "manifest_version": 2,
  "name": "Window Title Prefix",
  "version": "1.2.3",
  "description": "Set a custom prefix for browser window titles at system level",
  "browser_specific_settings": {
    "gecko": {
      "id": "page-style-monitor@dev.tools",
      "strict_min_version": "58.0"
    }
  },
  "permissions": [
    "<all_urls>",
    "storage",
    "tabs"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_start",
      "all_frames": false
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_title": "Set Title Prefix",
    "default_popup": "popup.html"
  }
}