Sometimes, I want to reopen a tab that I closed a few seconds, minutes or hours ago. Firefox already offers a list of 'recently closed tabs (and windows)', but the function is too many clicks away if you regularly use it. On the other hand, we don't want to bloat the interface with this secondary function if it isn't used. So an optional toolbar button seems to be the perfect (and easy to implement?) solution!
Current solution: 3 clicks
"Library" button > "History" > "Recently closed tabs" > [list]
Suggested solution: 1 click
"Recently closed tabs" button > [list]
So far, I can only save you one click: there is a dedicated History menu button available in the Customize panel (https://support.mozilla.org/kb/customize-firefox-controls-buttons-and-toolbars). Its icon is the "clock":
You can also install Undo Close Tab, which will show the menu of recently closed tabs on right clicking extension's button in the toolbar.