Unlike Internet Explorer or Google Chrome (Duplicate tab: CTRL + K), Firefox does not possess a dedicated feature to duplicate your tabs.
If you want to duplicate a tab under Firefox, while retaining the original one.
- Select the tab
- Press the CTRL button
- While holding it down, click on the Refresh button.
You can download and install the Duplicate tab Add-on
- This add-on will add a small button in the Firefox toolbar, allowing you to perform the aboved mentioned action.