Undo|Duplicate Tab — Sarah Av.

Undo a closed Tab(s) or create a duplicate of your current Tab with Undo|Duplicate Tab addon.

Some features:
1. Allows you to restore a closed tab(s).
2. You can easily make a duplicate of the current tab.
3. Toolbar popup UI has two easily accessible buttons (undo & duplicate).

Note 1: this addon only works with Firefox version 52 and upwards
Note 2: in order to report bugs, please fill the bug report form in the addon's homepage.