Sorry for the earlier mess-up, I wasn't familiar with how this review system works. I assumed that if a developer responded to a review, you could then respond to that response, but it appears not! I just want to confirm that New Tab Override, if it's configured to open "about:blank" as a new tab, should produce the same result when clicking the "+" button on the tab bar as entering "about:blank" in the address bar and pressing "enter" does. At the moment it doesn't seem to produce the same result. Is this by design? Thanks, Dave.
EDIT: Thanks Sören, that certainly explains the change of behaviour. It's unfortunate that my extension to automatically close empty pages does not now treat the empty pages generated by New Tab Override as it did before, but that's obviously not your fault! I've tried the about:config change, but unfortunately that seems to result in a new tab being the home page, not an empty blank tab, so it's not ideal either, but again not your problem! Many thanks again, cheers, Dave.
There was a change in Firefox 60 with the result that about:blank is no longer usable as new tab page for WebExtensions. I don't know if this was a intended change or a side effect of another change, a bug. But the only thing I was able to do was to implement a workaround so that a empty new tab still works - even if there are differences to about:blank in the tab / address bar.
But if you really want an empty new tab you can change browser.newtabpage.enabled in about:config to false and you don't need an add-on at all.