Although apparently not available in this moment, Firefox 67 is scheduled to arrive today all over the world. The new release of Mozilla's browser comes with private browsing mode extension controls, WebRender enabled for 5 percent of compatible systems, as well as various performance improvements and fixes.
https://www.notebookcheck.net/Firefox-67-0-rolls-out-with-WebRender-performance-improvements-and-more.421246.0.html