I'm writing an application that first tries to open a WebSocket connection (to make sure no others are open; address collision checking) before firing off a custom protocol that will launch a one-time WebSocket server with the address that the browser tells it. All communication is done over localhost and some arbitrary port number, say 3000. I'm not doing anything special, just attempting to open a WebSocket:

var socket = new WebSocket("ws://localhost:3000/MyApp/");
socket.onclose = function(e) { console.error(e); }

When testing in Chrome, the WebSocket will actually stay in the CONNECTING state for a little bit, which is ideal, since it gives us some time to actually launch the app through the custom protocol. But in Firefox, the WebSocket immediately closes with code 1006 and I can't figure out why.

I've tried changing the about:config network.websocket.timeout.open setting to be 1000 (from 20), but that doesn't help. I've also found this related post: Websockets - chrome and firefox differences?. That hasn't lead me to finding an answer, either.

What am I missing?

Update 11/16/21
I'm using the Dev Tools in Chrome and FF to check out the requests. The weird thing is that Chrome is actually sending a request header as you'd expect, but in FF, the request is completely empty (0 Bytes). Maybe this is a problem with FF not supporting debugging native WebSockets (no wrapper libraries in use)? Is there some FF setting that nixes the request? But even more confusing is that the browser would hit the close event without ever hitting the open event.

Update 11/17/21
I realize that maybe this has something to do with launching a Custom Protocol Handler? I noticed that it will wait a second to try and connect to a web server if no CPH is launched, but then when I do launch a CPH, that is when it immediately closes the WebSocket. The CPH is launched via a link targeting "_parent".