I did try it. It even works from a different VT directly logged in as root if I also specify DISPLAY:

    sudo -u desktopuser XDG_RUNTIME_DIR=/run/user/1000 DISPLAY=:0.0 firefox --new-window

If I now switch back to the VT running my desktop, a new Firefox window has opened.

Maybe your root terminal has other environment variables set that break it? A different DBUS session maybe?