I installed Ubuntu 18.04 on two systems and it resulted in two different behaviours of the GNOME shell when one application has more than two Windows.
For example Firefox (wanted behaviour):
1 open Window + 2nd private window open. In the Ubuntu dock the Firefox Icon now has two dots. When clicking the Firefox Icon a little preview opens where i can choose which one to open.
For some reason this doesn't work on my second installation of Ubuntu 18.04 and I couldn't find any settings regarding this. When having two windows open and clicking the icon in the dock it automatically opens the recently used Window.
I really don't want to re-install from scratch because it took me a long time setting the system up and everything else is working perfectly fine.