In Firefox you can also middle click the reload button to duplicate the tab.
Edit: other things you can middle click:
- The back and forwards buttons: does, what the buttons do in a new tab, while duplicating the tab history.
- The new tab button: open a new tab directly after the current tab (and in the same container as the current tab).
- Items in the URL bar (such as autocomplete suggestions, the thing you just typed, or your frequent pages): opens them in a new tab (at the end though for some reason)
You can also Ctrl+click all of these for the same result.




And you can hide the close button by adding
to your userChrome.css file. Works with vertical tabs too.