Linux user using KDE with a Dark Arc theme. Firefox didn't line up well enough with theme changes but this one made it blend seamlessly with the rest of my setup. Thanks to the developer(s)

My absolute favorite theme for every day use. Clean, efficient, useful.

For anyone who prefers a light background for the new tab view, install version 19.0 from here https://addons.mozilla.org/en-US/firefox/addon/arc-darker-theme-we/versions/ and disable automatic updates for this add-on.