How to run Firefox from systemd service?

Question

Can’t figure out how to fix /system.slice/unsplashpi.service is not a snap cgroup error.

$ cat /etc/systemd/system/unsplashpi.service
[Unit]
Description=unsplashpi
After=multi-user.target
[Service]
Type=forking
ExecStart=/home/ubuntu/run.sh
User=ubuntu
WorkingDirectory=/home/ubuntu
[Install]
WantedBy=multi-user.target
EOF

$ cat /home/ubuntu/run.sh
#! /bin/bash

startx /home/ubuntu/unsplashpi.sh &> /home/ubuntu/startx.log &

$ cat /home/ubuntu/unsplashpi.sh
#! /bin/bash

DISPLAY=:0 firefox --kiosk --private-window --width 1080 --height 1920 "https://askubuntu.com" &

while :
do
  sleep 60
done

$ cat /home/ubuntu/startx.log
…
/system.slice/unsplashpi.service is not a snap cgroup