Wt

When the client closes a WebSocket connection it sends a message with a close opcode.

According to RFC 6455 5.5.1: If an endpoint receives a Close frame and did not previously send a Close frame, the endpoint MUST send a Close frame in response.

Not sending the Close frame back would cause Firefox to exponentially back off. The result would be that if you would close many tabs at once with open WebSocket connections to the same endpoint, Firefox would immediately set the delay for new WebSocket connections to 1 minute. See this issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1608701