Yes, tabs have arrived in File Explorer in Windows 10. If one of your Windows 10 computers are Skip Ahead testing the next version of Windows 10, you can have a quick taste of it and see how it works.
First of all, clicking the + icon next to the tab will not open another File Explorer tab. Instead, it opens an Edge browser tab staying there side-by-side with File Explore tab.
To open a new tab so you can have two File Explorer tabs grouped together in one Windows, simply press Ctrl + T, the standard keyboard shortcut in all tab-based browsers, or Ctrl + N which opens a new window.
To close the current tab, use Ctrl + W. Again, it’s also a standard keyboard shortcut to close a tab in browsers.
You can also open a new tab from the File Menu. Click File > Open new window > Open new tab.
Moreover, you can right-click any file folder and choose Open in new tab from the context menu to open the folder in a new tab in File Explorer.
How about the middle-button on your mouse? Unfortunately, that still doesn’t work. Will have to wait and see if Microsoft adds it in the future builds.
Just like how the tabs work in the browser, you can right-click any of the tabs to bring up the tabs context menu, from which you can close the current tab, close other tabs, close tabs to the right, or move the tab to the new window.
One last tip, there is a new setting in Settings app that controls how Alt+Tab behaves, allowing the tabs to show in Alt+Tab view or not.
Open Settings, go to System > Multitasking and change the setting in Sets section.