Hot answers tagged lightdm
I'd personally side-step all the lightdm and Xsession stuff and just launch my own instance of X that just started Firefox. Sounds nightmarish but it's really simple with a little Upstart script description "Starts X and Firefox" start on (filesystem and stopped udevtrigger) stop on runlevel  task console output emits starting-x respawn exec su -c ...
Both will work fine. You don't need both installed as gdm is a recommends dependency of gnome-shell, not a hard dependency. You can uninstall one or the other depending upon how you like them. I have used both with gnome-shell and faced no issues because of them.
Fortunately I had a working system to compare it to. Ubuntu Tweak for some reason deleted a symlink in /etc/X11 and this caused lightdm to fail. The fix is easy: cd /etc/X11 sudo ln -s /usr/bin/Xorg X and Bob's yer uncle.
If you can't login normally because a file 50-unity-greeter.conf has been impaired, you can try this: PressCtrl+F6 Input Login and your password Type startx Open any directory and go to a hardDisk and search greeter. When you see for example: unity-greeter_14.04.10-0ubuntu1_amd64.deb, or anything like greeter.deb, execute this file. reboot and that's ...
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