As documented in NoVNC under known issues:

when using Firefox, if you use foreman via https, firefox might block the connection (see limiations below), to fix it, goto about:config and enable network.websocket.allowInsecureFromHTTPS

Foreman defaults to https:// only, so every firefox user runs into this. It would also increase security if all console access was encrypted. Websockify does support it, so it's mostly a matter of passing the correct parameters. Ideally the used certificates would be overrideable through settings.yaml.