RPI2 Model B V1.1 no automatic start of LXDE

Hello,
I want to automatically start Firefox with a custom address.

For this I have installed with Dietpi software > 23 LXDE ultra lightweight desktop < included Firefox.
The Raspi features a Wi-Fi stick and an original touchscreen from the Raspberry Foundation.

Raspi drives up to the login window
manually logged in with root and password
Manually started with > startx < LXDE
Manual Firefox started with a custom address

Everything works manually without problems.

This configuration is backed up with an SD card backup.
I always have a defined starting situation.

How can I automate the above steps?

I got with Dietpi - Autostart
16 LightDM login mask
2 Automatic login
installed.

After a restart of the Raspi, it drives up, then there is only a dark screen.
There is no error message and no way to get to the console.
It only helps draw plugs.

What am I doing wrong?
Does anyone have any hints?
Thank you.