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.
Best Answer
Open Terminal and run the following command on your second installation
This would provide the little preview windows you're looking for.