Firefox 3 will let you duplicate currently opened tabs. This may come handy for those times when you need to split navigation but need to keep a copy of where you started, or are about to take action on a web page that may change it and want to keep a reference.
Whatever the reason, you will just have to press the Ctrl (Cmd on Mac) key while dragging the tab you want to duplicate and dropping it where you want it to appear. To visually differentiate from plain old tab moving, the mouse cursor changes to a copy cursor with a cross.
So far, there is no user interface such as main menu or context menu item to duplicate a tab. Also, note that the duplicated tab doesn’t inherit the original tab’s history.
Noticed the ghost tab in the screenshot? It is a new visual trick for all dragged elements and will also be available for massive consumption in Beta 2, later this month.
If you want this feature now, you may want to try Duplicate Tab, a Firefox extension that provides this same feature for Firefox 2.