Examine source code of Mask Everything - Subtitle

{
  "manifest_version": 3,
  "name": "Mask Everything - Subtitle",
  "version": "1.2",
  "displayName": "Mask Everything - Subtitle",
  "description": "A versatile browser extension allows users to easily mask any content on the page for aiding language learning, such as subtitle.",
  "author": "azimuth",
  "permissions": [
    "storage",
    "activeTab",
    "scripting"
  ],
  "host_permissions": [
    "https://*/*",
    "http://*/*"
  ],
  "action": {
    "default_popup": "popup/popup.html",
    "default_icon": {
      "16": "logo.png",
      "48": "logo.png",
      "128": "logo.png"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "browsermask@til19630.com"
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "run_at": "document_end",
      "js": [
        "content.js"
      ]
    }
  ]
}