It works if you use -E both when leaving the user environment and when reentering it. There's something in the environment variables that needs to be set for the new firefox instance to communicate with the old one to pass along the new window command, and using -E both times carries it along.

Also ssh is somehow able to load this from somewhere without carrying it along. You can "ssh desktopuser@localhost" to restore it, though this does mean you need to set "DISPLAY=:0" manually.

I am working from memory, on my smartphone in public transit in a heatwave.

I'd sacrifice some karma but I am hesitant so go see yourself out and off to https://stackoverflow.com instead.