Note 1: You may run in a Firefox bug which renders Undo Close Tab unusable. If this happens to you, you can follow the following instructions to revive the Addon: How to fix broken browser session
Note 2: The default setting in Firefox is to only store information for a maximum of 10 closed tabs. If you want to reach the maximum of 25 tabs, which is a limitation of the WebExtensions API, then you have to increase the value of the browser.sessionstore.max_tabs_undo preference via about:config.
Main repository: https://github.com/M-Reimer/undoclosetab.
AMO: https://addons.mozilla.org/firefox/addon/undoclosetabbutton/
Localization: https://lusito.github.io/web-ext-translator/?gh=https://github.com/M-Reimer/undoclosetab/
Hacking: Do a temporary install.
Building: make