I’ve been using “Duplicate Window 2” for some time now with flawless results. Today, however, it’s opening a new Firefox 44.0.1 window, but of my home page.
I cleared cache, restarted FF, went to the URL I wanted to clone, and tried again with same result. Then I Removed the Add-on, cleared cache again, went to “get add-ons” and reloaded Duplicate Window 2, installed it successfully, cleared cache once more, and restarted FF again.
Navigated to original URL then did . No change. New window still goes to home page.
I see you filed an issue on their GitHub. Since that’s the only public contact info they have, I don’t think there’s much more to do than wait. The developer looks active on GitHub, so it’s probably just a matter of time.