When you are typing a duplicate URL, you can hold the SHIFT key while pressing Enter to open that dupe URL without switching over to the other tab that contains it.

Not sure this is what you wish but you can duplicate a tab. In much the same state as the original. So for instance you could duplicate the tab you write these messages in, complete with the text message content.

Use Control+Drag with the tab.

Probably what you are trying can be done in two steps.

1. Open a tab (Ctrl+T or just use an existing tab)
2. Type or paste the address into the address bar. Hit enter or click the arrow.
   Just ignore the offer to switch tabs.

Note the SHIFT method works but it opens a new window.

John,

Really? I must be doing something different then? When I am, for instance, on Allizom. I have the original Allizom site open, CTRL + T, then start typing support.allizom, it pulls it up, says "Switch to tab", I highlight that, SHIFT + ENTER gives me a copy...

Moses,
Curious, a bit of a mystery. I wonder what others see ?

Maybe I am missing something myself or doing something different.

I did wonder if it was one of my settings or the Ubuntu build, but I have just tried in Mozilla Nightly with a clean profile.

This is what I have tried, and what I see

1. Open support.allizom.org
2. Open another tab (Ctrl+T)
3. Drag the original support.allizom.org to get it in its own window (W2)
4. In a tab in the original window (W1) start typing support.allizom
5. Note the support title (support.allizom) appears as a "Switch to tab" option.
6. Select & Highlight that option.
7. Press & hold shift:
   This displays the full url instead of the "Switch to tab" icon
8. Press enter:
   This opens support.allizom.org in a brand new Window (W3).
   (Alternatively left click on the url that appears; instead of hitting Enter)
   I now have 3 Nightly windows open.

I have also tried to rule out that it is a recent change in behaviour or Fx31 regression and tried similar in ESR 24.4.0
I have not tried checking Windows 7 in case Windows behaves differently to Linux, although no reason to expect it to.

Works for me on Linux with the Enter key and Shift and Alt with an entry in the location bar drop-down list highlighted.

When I click the mouse the the behavior is the same as left-clicking a link with the exception that you need to hold Shift and Ctrl to open the URL in a new tab as Shift needs to be held down to disable switch-to-tab.
So holding Shift and left-clicking a link opens in a new window.
Holding Alt and left-clicking an URL is determined by the value of the browser.altClickSave pref and holding Alt and left-click opens the Download window.
This even works on Linux (!) where normally nothing happens when you hold down Alt and left-click a link (Alt is not a supported modifier for clicking the mouse on Linux).

In response to one of your original questions, there is no built-in way to disable Switch to Tab so, of course, someone created an extension: https://addons.mozilla.org/firefox/addon/switch-to-tab-no-more/