Firefox has built-in "Ctrl + Shift + T". This works even without any extension installed.
But you can create an additional shortcut "calling into" my extension using:
https://support.mozilla.org/en-US/kb/manage-extension-shortcuts-firefox

There are people out there that find it convenient to just do one click instead of holding three keyboard keys. Especially as you have the mouse in your hand while browsing anyway. That's the only reason why this Add-on exists. If you don't want to have a button in your toolbar (which, by the way, has the recently closed tabs as a list in its context menu), then you are right: In this case there is no need to install my Add-on.

Warum das "leider" kann ich an der Stelle nun leider nicht nachvollziehen.
Das Ziel war Firefox-Funktionalität (ja, die ist schon "ab Werk" da) leichter zugänglich zu machen. Wer mit der Zugänglichkeit der "Standard-Funktionen" zufrieden ist, der kommt aber auch gut ohne mein Add-on klar.
Trotzdem danke für die Rückmeldung. Ich wünsche schon mal frohe Feiertage!

No. You can only define a new hotkey which has to be different from Ctrl+Shift+T. If you need further assistance, then please open an Issue on GitHub.

The key bind is included in Firefox by default. Just try "Ctrl + Shift + T".
If all you really need is a key bind, then you don't need any Add-on at all.