firefox usage restarts kde

I have upgraded from 10.3 to 11.3 recently.
After removin /etc/X11/xorg.conf the system overall runs fine.

When using firefox kde sometimes restarts with a popup referencing file /opt/kde3/share/apps/themes/SUSE.
How can I prevent this? Is this a 10.3 relict that hides in some configuration?

I see this is your first post here. WELCOME.

I do not know if you have spend some time before posting, in searching for threads about going from one openSUSE level to the next (or even skipping levels).

And I wonder what you mean by “upgraded”. There is no supported upgrade path from 10.3 to 11.3. You should do a fresh install (of course saving your user data that is hopefully on a separate partition like /home). I doubt that the fresh install would have created a /opt/kde3 directory, let alone what is inthere.