That could be a hard one as Mozilla really makes it difficult to integrate with the existing themes.
But I maybe found something that is worth trying. Can you create a GitHub issue for this?

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