Hold down the control key, then drag the tab that you want to duplicate onto the tab bar.

Firefox 4 uses a combined Stop/Reload/Go button that is positioned at the right end side of the location bar.

Middle-click the Reload button to duplicate the current tab to a new tab.

Both of the above replies assume you are well aware of the other window pointing to the URL that you are about to open. If you're not, Firefox closes your current window and switches to the other one, where it updates the content.

I frequently have a specific URL which is displaying information from a particular time or place within the URL. I haven't read it yet, but because I only have dial-up speed, I have been doing other things while the page downloaded (like taking a long nap).

So I come back to the computer, bring up a new Firefox window, and go to the same URL again. Bam! The other window has just been clobbered, along with all the info I haven't read yet.

Silly example: the URL is that of a local newspaper. I download the front page, but before I get a chance to read it, the sun goes down and I go to sleep while my computer hibernates. The next day, we both wake up and I decide to go to the newspaper again. Ignoring the many open Firefox windows waiting to be read, I open a new Firefox window and go to the newspaper. The current day's front page replaces yesterday's, which I hadn’t read yet and would have liked to.

Another example: I want to open the same URL in several different windows and then use each of them separately to search for different things within that URL.

So how do I get Firefox to open the same URL in more than one window?

If you hold down the Shift key and left-click a link or bookmark then the link opens in a new window. Or you can open a new window and paste the link in the location bar in that new window.

If you have a problem with "Switch to Tab" then try one of these:

• Press the Shift key while an entry in the drop down list is highlighted to disable "Switch to Tab" and see the URL.
• Use the cursor Right key to select an item marked as Switch to Tab in the drop down list and copy the URL to the location bar.
• Open the URL in a new tab by using Alt + Enter on the highlighted entry.
• Open the URL in the current tab by using Shift + Enter on the highlighted entry.