[jorams]

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?

[JonathanBeuys]

Oh wow, you were right! This solves it:

    sudo -u desktopuser XDG_RUNTIME_DIR=/run/user/1000 firefox

Sorry for messing it up the first time I tried!

Man, you solved it!!

Thanks a 1000 times!

[jorams]

No problem! Glad it works now.