Appears to start by default with a Gnome session:
user@host:~$ ps aux | grep fwup[d]
root 2743 0.1 0.1 555484 18160 ? Ssl 01:05 0:00 /usr/lib/fwupd/fwupd
user@host:~$ systemctl | grep fwupd
fwupd.service loaded active running Firmware update daemon
Doesn’t seem to start from a fresh boot into a KDE/Plasma session, but will continue running if you start a Gnome session and log out.
Seems like a strange approach to handling firmware updates. I’ll stick with Plasma and avoid fwupd for now until I know more about it.