Duplicates the current tab when you click the icon. This is a firefox web extension: https://addons.mozilla.org/en-GB/firefox/addon/duplicate_tab/
The main focus of this project has moved to keyboard shortcuts now that Firefox 57 supports a right click menu by default. From Firefox 60 you can customize these shortcuts in browser.
This project is licensed under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version or version 2.