Hmm, apparently Discord uses the WebSocket (ws:) protocol to connect to the Discord app listening on a particular port your system. Off the top of my head, I don't know how to selectively disable WebSockets in private windows, or on particular sites. Perhaps there is an add-on that can do it?? ---- For sites that use a custom protocol (such as zoommtg://) to trigger an external application, Firefox 84 added a new site-specific permission for protocols that launch external applications. There are now two layers of potential approvals: (A) This site (B) Whether Firefox launches the app automatically or asks you every time So I'm pretty sure -- based on your description and some web searching -- that Discord is not using that method and those protections do not apply.