Examine source code of Volume Master

{
  "content_scripts": [
    {
      "matches": [
        "http://*/*",
        "https://*/*",
        "file:///*"
      ],
      "all_frames": true,
      "run_at": "document_start",
      "js": [
        "lib/arrive.min.js",
        "cs.js"
      ],
      "match_about_blank": true
    }
  ],
  "commands": {
    "_execute_action": {
      "description": "Volume booster firefox extension that remembers your choices"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "strict_min_version": "58.0",
      "id": "{0dc299d7-8f55-4584-b466-7f9c319c6561}",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    },
    "gecko_android": {
      "strict_min_version": "92.0"
    }
  },
  "description": "Adds a volume control for the current site, that can also boost the volume beyond the normal range.",
  "icons": {
    "96": "icon-128.png"
  },
  "host_permissions": [
    "<all_urls>",
    "*://*/*"
  ],
  "options_ui": {
    "page": "options.html",
    "open_in_tab": true
  },
  "version": "1.7.0",
  "manifest_version": 3,
  "action": {
    "default_icon": "icon-128.png",
    "default_title": "Volume Control",
    "default_popup": "popup.html"
  },
  "permissions": [
    "storage",
    "activeTab",
    "declarativeNetRequest"
  ],
  "name": "Volume & Sound Booster"
}