systemd unit to start a browser/user process on network up

It took me far too long to get this script right. The gottchas:

1. User units cannot reference or interact with system units.
2. On my system at least I needed to specifically enable systemd-networkd-wait-online.service to have network-online.target work.
3. To enable pulse audio from a system process you need to
   1. set XDG_RUNTIME_DIR, which
   2. needs uid, not user name.

[Unit]
Description=Start Firefox
Wants=network-online.target
After=network-online.target

[Service]
User=[USER]
Environment="DISPLAY=:0"
Environment="XAUTHORITY=/home/[USER]/.Xauthority"
Environment="XDG_RUNTIME_DIR=/run/user/[UID]"
ExecStart=/usr/bin/firefox

[Install]
WantedBy=multi-user.target


I haven't looked into using this unit as a template as I'm not sure how to translate the user name into uid for the XDG_RUNTIME_DIR variable. It's a real shame we can't run it as a user process else we'd be able to use %u and %U.