Duplicate Tab, is a Firefox extension developed by Twanno, that adds three items to the tab context menu to let you duplicate a currently opened tab to a new tab or window.
Duplicated tabs even inherit the original tab’s history so you can move back and forward in the duplicate tab as it was the original.
You can also drag and drop items by pressing Ctrl while dragging and dropping the tab to the desired location.
As a bonus, it also allows to merge all opened or current window into a single one.
It’s not short of customization options: you can make make duplicate tab the default action for middle clicking on links, define the number of history items to inherit when duplicating tabs, where to place the duplicated tab and what to show in the tab context menu.
Get Duplicate Tab from Mozilla Add-ons.