Safari 11.0.1. Before the update to High Sierra I could open a second tab in Safari and I would not lose any of the other tabs I had open. For instance, I have email open, if I then opened facebook a new tab would appear for facebook and the tab for email would remain (it would be moved to the side). Now when I open the second tab (facebook) its tab REPLACES the tab for email. Apple support told me to open in safe mode and if that didn't work to go to system preferences (on the mac), users and groups and remove the login items. I feel it must be something in Safari.