Works well. It would be nice to have an option to make it work in another way, so that the new tab is opened next to the current tab, while the old one gets closed.
I've just released an update which includes advanced options and several improvements to the default behavior - please check it out (and feel free to open an issue on GitHub if you have any further suggestions: https://github.com/sonnenfell/firefox-new-duplicate-tab-closer/issues)
