[Unit]
Description = Firefox
# Requires=ioslay@%I
# Requires=pulseaudio.socket # require X11 somehow
ConditionUser = !root # seems reasonable
ConditionEnvironment = DISPLAY

[Service]
ExecStartPre = /usr/bin/systemctl --user start ioslay@%I
ExecStart = /usr/local/bin/firefox --new-instance --display=%I
ExecStopPost = /usr/bin/systemctl --user stop ioslay@%I
Restart = on-failure
PassEnvironment = DISPLAY XAUTHORITY
MemoryMax = 50%

[Install]
Also=ioslay@.service
WantedBy = default.target # make an X11 user target?