Good concept, have used in the past but removed for now. As others have noted, it opens the New Tab settings page rather than the selected New Tab. Also, removing the add-on didn't work.

Update to reply to developer response: "New Tabe Override still works." I'm giving you feedback that it doesn't, I could give you more information (Firefox on Dell laptop running Win 10 etc.) but a blanket response like that is odd. Why would I bother leaving this comment if I'd not had an issue?
To clarify "removing the add-on didn't work" in response to your answer - I just clicked on the remove button and the removal path (menus etc.) appeared but the add on was still present on Firefox. I did this a couple of times before it eventually went. Other add-ons have been successfully removed first time. I mentioned this because it seemed like useful feedback, I'm not smart enough to either develop apps or know whether it's possible for them to have an issue that can interfere with the removal process, although I have worked alongside software testers for enough years to believe that all kinds of unexpected behaviours can sometimes occur with software, so any feedback is usually useful in case the issue is a defect that impacts others, rather than just a temporary glitch or problem with the machine or browser.
It certainly could be something broken on my system, I'll be on the look out for any other apps not removing properly, thanks for the tip.