When entering an URL that you already have opened, Firefox will instead of opening a new tab go back to that tab where the URL was already open. So you can't open duplicates.
How can I disable this functionality?
Go to Solution.
I previously had Omnibar installed, but now also installed Omnibar Plus. That seems to have taken away the annoyance of having Switch to as the first alternative.
So I recommend these two addons which also makes the address bar superior! Just like the excellence of Google Chrome.
When the location bar dropdown is showing that you can 'Switch to Tab', you can override it with the Shift key (opens the url in the current tab) or the Alt key (opens the url in a new tab).
You can also duplicate a tab by holding Ctrl and clicking a Reload button or menu item. The middle mouse button works there too.
I don't think there's a preference to disable the Switch To Tab feature entirely, but this add-on will: https://addons.mozilla.org/en-US/firefox/addon/switch-to-tab-no-more/