It does not work with FF prior to version 56.
Interestingly enough, Mozilla did not remove this Legacy add-on(that does not work) from their site, but instead they removed this one(Copy URL): https://legacycollector.org/firefox-addons/252534/index.html , that works flawlesly, when pressing the button on the toolbar, it simply and easily,quickly copies the active's tab URL(web page address) to clipboard.

Странно на ноутбуке расширение работает (копирует ссылку и ОК высвечивается на кнопке), а на рабочем компе не копирует..