JS Injector

manifest.json

{
  "manifest_version": 2,
  "name": "JS Injector",
  "short_name": "JS{I}",
  "description": "Inject JavaScript custom code into every webpage (e.g. to help with debugging).",
  "version": "1.0",
  "permissions": [
    "storage",
    "activeTab"
  ],
  "icons": {
    "16": "assets/icon16.png",
    "48": "assets/icon48.png",
    "128": "assets/icon128.png"
  },
  "browser_action": {
    "default_icon": {
      "16": "assets/icon16.png"
    }
  },
  "background": {
    "scripts": [
      "js/lib/jquery-3.4.1.js",
      "bg.js"
    ]
  },
  "applications": {
    "gecko": {
      "id": "@jsinject",
      "strict_min_version": "57.0"
    }
  },
  "options_ui": {
    "page": "options.html",
    "browser_style": true,
    "open_in_tab": true
  },
  "content_scripts": [
    {
      "matches": [
        "*://*/*"
      ],
      "run_at": "document_end",
      "js": [
        "content.js"
      ]
    }
  ]
}