Running graphical applications from emacs

[Felix Weilbach]

Hello,

if I try to run a graphical application like firefox from emacs with for 
example shell-command or from shell, ehsell etc. I get the following error:

Unable to init server: Verbindung ist gescheitert: Verbindungsaufbau 
abgelehnt

Error: cannot open display :1


The first error translated from german to english means Connection 
failed: connection refused

If I open a normal shell and execute echo $DISPLAY, I get :0. However if 
I execute the command in emacs with DISPLAY=:0 firefox, I get same error 
Error: cannot open display :0.

I'm running on Arch Linux with KDE 5.17.2 on X11. My emacs version is 
27.0.50.

Thanks in advance for any help with this problem.

Felix