Simple, but time saving button. Dont know, why Mozilla is so stupid, not having this as an standard option.

stopped working after latest ff update (ff 99 on M1 macbook)

Has saved me from losing unintentionally closed tabs numerous times.

It's worth adding a screenshot for the options because I thought it only offered a menu through the icon and then saw there was also an option to have it in the tab's context menu (which is how I was used to from Undo Closed Tabs Menu, but its functionality broke a bit recently so I switched)