I am a Mozilla supporter and FF is my "daily driver" browser. Very interested in this feature. Chrome has had it for a years, and it's a killer feature for many developers. It's very very useful to have multiple browser windows open, each logged into the same site as a different user. A lot of people do this by opening multiple browsers (FF, Chrome, Edge, Safari, etc) but that has its limits and it just adds another…
Firefox has had multiple profiles for years too, though the UI isn't as nice as Chrome's. This is different, this is basically profiles that can sharethe same browser window.
/Applications/Firefox.app/Contents/MacOS/firefox-bin --profilemanager