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.