Here is a quick and easy keyboard shortcut to duplicate or clone the current open tab in Firefox 4.
Simply click on the address bar for focus (you’ll note that the address highlights) and then press Alt+Enter to open that same address in a new tab. Nifty.
Note that you can achieve a similar affect by holding down Ctrl and clicking on the browser Reload button (yes, yes, I know that this isn’t a keyboard shortcut). This is of course because the default action when holding down Ctrl and left-clicking on any URL embedded in a page is to open that URL in a new tab.