Use border instead of ::before and ::after for placeholders because they

would make Fx to ignore nav-bar overflow

chrome/hide_tabs_with_one_tab.css

@@ -42,9 +42,6 @@
 :root[tabsintitlebar] #toolbar-menubar[autohide="true"][inactive]{
   transition: height 0ms steps(1) 80ms;
 }
-#nav-bar::before,
-#nav-bar::after{
-  content: "";
-  display: -moz-box;
-  width: var(--uc-window-drag-space-width);
+#nav-bar{
+  border-inline: var(--uc-window-drag-space-width) solid var(--toolbar-bgcolor);
 }

chrome/navbar_tabs_oneliner.css

@@ -43,6 +43,7 @@ urlbar_full_width.css is VERY MUCH recommended for Firefox 71+ because of new ur
 }
 
 /* Override style set in window_control_placeholder_support.css  */
+#nav-bar{ border-right-width: 0px !important }
 #nav-bar::after{ display:none !important }
 
 /* Rules specific to window controls on right layout */

chrome/navbar_tabs_oneliner_tabs_on_left.css

@@ -52,6 +52,7 @@ Window controls will be all wrong without it
 }
 
 /* Override style set in window_control_placeholder_support.css  */
+#nav-bar{ border-left-width: 0px !important }
 #nav-bar::before{ display:none !important }
 
 /* Rules specific to window controls on right layout */

chrome/navbar_tabs_responsive_oneliner.css

@@ -32,6 +32,7 @@ Window controls will be all wrong without it
   }
 
   /* Override style set in window_control_placeholder_support.css  */
+  #nav-bar{ border-right-width: 0px !important }
   #nav-bar::after{ display:none !important }
 
   /* Rules specific to window controls on right layout */

chrome/window_control_placeholder_support.css

@@ -12,20 +12,16 @@
   --uc-window-drag-space-width: 24px; /* Extra space reserved on both sides of the nav-bar to be able to drag the window */
 }
 
-#nav-bar::before,
-#nav-bar::after{
-  content: "";
-  display: -moz-box;
-  width: var(--uc-window-drag-space-width,0);
+#nav-bar{
+  border-inline: var(--uc-window-drag-space-width,0px) solid var(--toolbar-bgcolor);
+  border-right-width: calc(var(--uc-window-control-width,0px) + var(--uc-window-drag-space-width,0px));
 }
-toolbar#nav-bar::after{ width: calc(var(--uc-window-control-width,0px) + var(--uc-window-drag-space-width,0px)) }
 
 /* Use this pref to check Mac OS where window controls are on left */
 /* This pref defaults to true on Mac and doesn't actually do anything on other platforms. So if your system has window controls on LEFT side you can set the pref to true */
 @supports -moz-bool-pref("layout.css.osx-font-smoothing.enabled"){
 	:root{ --uc-window-control-width: 72px; }
-  :root[tabsintitlebar="true"]:not([inFullscreen]) #nav-bar::before{
-    width: calc(var(--uc-window-control-width,0px) + var(--uc-window-drag-space-width,0px))
+  :root[tabsintitlebar="true"]:not([inFullscreen]) #nav-bar{
+    border-inline-width: calc(var(--uc-window-control-width,0px) + var(--uc-window-drag-space-width,0px)) var(--uc-window-drag-space-width,0px)
   }
-  :root[tabsintitlebar="true"]:not([inFullscreen]) toolbar#nav-bar::after{ width: var(--uc-window-drag-space-width,0px); }
 }