WebSocket monitor is an extension for Firefox developer tools that can be used to monitor WebSocket connections in Firefox. It allows to inspect all data sent and received.
Support for Socket.IO and SockJS included.
https://github.com/firebug/websocket-monitor/wiki
You need Firefox 44 (DevEdition or Nightly)
https://www.mozilla.org/en-US/firefox/developer/
This extension isn't signed, so you might need to set xpinstall.signatures.required pref to false.