MQTT client connection fail in firefox

I try to connect with websockets to my broker (mosquitto). This works in Chrome (and in Edge), but in Firefox I get following error:

Firefox can’t establish a connection to the server at ws://:/ws.

Then, the onFailure callback gets called with this parameter:
Object { invocationContext: undefined, errorCode: 7, errorMessage: "AMQJS0007E Socket error:undefined." }