--- a/browser/themes/shared/tabs.inc.css
+++ b/browser/themes/shared/tabs.inc.css
@@ -546,21 +546,16 @@ tabbrowser {
 
 /* Tab separators */
 
 .titlebar-placeholder[type="pre-tabs"],
 .titlebar-placeholder[type="post-tabs"] {
   width: 40px;
 }
 
-.titlebar-placeholder[type="pre-tabs"] {
-  border-inline-end: 1px solid;
-  opacity: 0.2;
-}
-
 .tabbrowser-tab::after,
 .tabbrowser-tab::before {
   border-left: 1px solid;
   /* Vertical margin doesn't work here for positioned pinned tabs, see
      bug 1198236 and bug 1300410. We're using linear-gradient instead
      to cut off the border at the top and at the bottom. */
   border-image: linear-gradient(transparent 6px,
                                 currentColor 6px,
@@ -598,16 +593,17 @@ tabbrowser {
   border-image: linear-gradient(transparent 1px,
                                 var(--tabs-border) 1px,
                                 var(--tabs-border) calc(100% - 1px - var(--tab-toolbar-navbar-overlap)),
                                 transparent calc(100% - 1px - var(--tab-toolbar-navbar-overlap))) 1 !important;
   opacity: 1;
 }
 
 .tabbrowser-tab:not([first-visible-tab])::before,
+#main-window:not([inFullscreen]) #tabbrowser-tabs:not([overflow]) .tabbrowser-tab[first-visible-tab]::before,
 #tabbrowser-tabs:not([overflow]):not([movingtab]) > .tabbrowser-tab[last-visible-tab]::after,
 /* Also show separators beside the selected tab when dragging it. */
 #tabbrowser-tabs[movingtab] > .tabbrowser-tab[visuallyselected]::after {
   content: "";
   display: -moz-box;
 }
 
 /* Tab bar scroll arrows */

--- a/browser/themes/windows/browser.css
+++ b/browser/themes/windows/browser.css
@@ -777,16 +777,21 @@ html|span.ac-emphasize-text-url {
 #TabsToolbar {
   min-height: 0;
   padding: 0;
   margin-bottom: calc(-1 * var(--tab-toolbar-navbar-overlap)); /* overlap the nav-bar's top border */
 }
 
 %include ../shared/tabs.inc.css
 
+/* Don't show the starting border of the first tab when maximized */
+:root:not([sizemode="normal"]) .tabbrowser-tab[first-visible-tab]::before {
+  border: none;
+}
+
 /* tabbrowser-tab focus ring */
 .tabbrowser-tab:focus > .tab-stack > .tab-content {
   outline: 1px dotted;
   outline-offset: -6px;
 }
 
 /* Tab DnD indicator */
 .tab-drop-indicator {