I have an issue with the images not appearing on the New Tab Page in Firefox 60 now, but I haven't taken much time trying to sort that out. As far as about:config changes, the prefs.js file in the Profile folder shows all the prefs that are 'userset' which includes prefs set by extensions. Any modification you did with 57, 58, or 59 to the New Tab Page may have caused that 'gear' to not appear in 60 either via an add-on, via userChrome.css mods or by changing prefs in about:config so look at those two areas.
