Firefox 67+ always opens external links in a new tab or new window in your default profile, running or not. If you use Firefox's external Profile Manager (the classic dialog, not the about:profiles page), that could explain the behavior you see because I think that dialog changes your default profile to the latest one you opened. I don't know what is possible with command line or Registry tweaks... probably not that much. Maybe it's possible to create an external URL handler to manage the process of intercepting URLs, focusing the desired window, creating a new tab, and navigating it to the URL.