Examine source code of Media Keys Ultra

{
  "manifest_version": 3,
  "name": "Media Keys",
  "version": "1.0",
  "description": "Increases/decreases video playback speed using Alt+Up and Alt+Down.",
  "browser_specific_settings": {
    "gecko": {
      "id": "mediakeys@firefox-add-on"
    }
  },
  "permissions": [
    "activeTab",
    "scripting"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "commands": {
    "increase-speed": {
      "suggested_key": {
        "default": "Alt+Up"
      },
      "description": "Increase video playback speed"
    },
    "decrease-speed": {
      "suggested_key": {
        "default": "Alt+Down"
      },
      "description": "Decrease video playback speed"
    }
  }
}