First, I am using Firefox 54.0.1. I successfully installed the Rename Tab Title add-on. But when I go to Customize (to add the Rename Tab Title icon to the toolbar), the icon is not there. Removed and reinstalled the add-on. No change. So sure, a small slice of WTF.

Secondly, couldn't you not adjust (modify) your code, making it possible to right click on the tab and say Rename?