Go to the Mozilla Add-ons Web Page {web Link} (There’s a lot of good stuff here) and search for what you want.

Thanks for the response. Been there, done that - no joy.

Found some links with a web search but they all went to Mozilla "Oops!" pages.

This is a feature that should be present "out of the box".

Went to uninstall, was given an option to Reset, now all is well

Also, found a keyboard shortcut that works: Control +Sift +T

That was very good work. Well Done.

Note that Undo Closed Tab isn't available when you are in Private Browsing mode, but it should still be present as grayed out. There are several places where you can find this item.

• History menu
• Tab context menu
• "List all tabs" menu

It is possible that an extension hides this item and replaces it with something else.