diff -up firefox-71.0/browser/base/content/browser.css.old firefox-71.0/browser/base/content/browser.css
--- firefox-71.0/browser/base/content/browser.css.old	2019-12-04 18:15:20.059908708 +0100
+++ firefox-71.0/browser/base/content/browser.css	2019-12-04 18:15:23.181905115 +0100
@@ -841,7 +841,7 @@ window[chromehidden~="toolbar"] toolbar:
 /*  Full Screen UI */
 
 #fullscr-toggler {
-  height: 1px;
+  height: 0px;
   background: black;
 }