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I'm getting my Ubuntu builds from Here. It appears that Firefly is using the lightdm desktop manager. I'm trying to create a NAS replacement device, so I don't want a desktop. After a bunch of digging, this appears to be the fastest was do disable the desktop:
sudo systemctl disable lightdm.service
sudo shutdown -r 0
Now my idle draw is 285mA which is on par with my RPi 3.
FYI: I read that Ubuntu Server and Ubuntu Desktop Ubuntu use the same kernel (https://www.makeuseof.com/tag/difference-ubuntu-desktop-ubuntu-server/).
Hope that helps!