well, i sucked that open a bookmark after migration to KDE5 with the middle mouse key no longer opens it in a new tab but instead overwrites the current one after the last update you can't even close a tab and when configured to show a close icon for every tab it also no longer works at all don't get me wrong but lose middle-click capabilities on a unix system is a joke https://bugzilla.redhat.com/show_bug.cgi?id=1796174
Updated Summary, to make clear this is about opening bookmarks via middle click. Right click -> open in new tab , still works, but middle click bookmarks seems to always open in the current tab.
and you can no longer close tabes with middle click since the latest update and the close icon in the tab itself (if enabled in the options) does nothing if you have open 20 tabs and want to close 10 specific ones it's terrible that you need to focus them and the move the mouse to the right edge on a 32" screen instead just press 10 times the middle key and be done