diff --git a/browser/themes/shared/tabbrowser/tabs.css b/browser/themes/shared/tabbrowser/tabs.css
index 61c0b8382c78f63ad9113af55a6236c4a29bc45b..0ab463c768d4fb3f45d22e753f408c46b61c0480 100644
--- a/browser/themes/shared/tabbrowser/tabs.css
+++ b/browser/themes/shared/tabbrowser/tabs.css
@@ -46,7 +46,7 @@
   --tab-group-line-thickness: 2px;
   --tab-group-line-toolbar-border-distance: 1px;
   /* Collapsed tabs should be square, so set width to match the min height */
-  --tab-collapsed-background-width: var(--tab-min-height);
+  --tab-collapsed-background-width: 48px;
   --tab-collapsed-width: calc(var(--tab-collapsed-background-width) + 2 * var(--tab-inner-inline-margin));
   --tab-collapsed-split-pill-offset: 0px;
   --tab-pinned-min-width-expanded: calc(var(--tab-pinned-expanded-background-width) + 2 * var(--tab-pinned-margin-inline-expanded));
@@ -308,7 +308,6 @@ tab-split-view-wrapper[dragtarget] {
   }
 
   :root:not([uidensity="compact"], [sidebar-expand-on-hover]) &[pinned] {
-    padding: 0 calc((var(--tab-min-height) - var(--icon-size)) / 2);
   }
 
   &:is([selected], [multiselected]) {
@@ -322,6 +321,7 @@ tab-split-view-wrapper[dragtarget] {
     border-radius: inherit;
     position: relative;
     overflow: hidden;
+    display: none;
 
     &::before {
       position: absolute;
@@ -519,10 +519,6 @@ tab-split-view-wrapper[dragtarget] {
 
   @media -moz-pref("browser.tabs.fadeOutUnloadedTabs") {
     &[pending] {
-      filter: grayscale(100%);
-      @media (prefers-color-scheme: dark) {
-        filter: grayscale(100%) invert();
-      }
       opacity: 0.5;
       /* Fade the favicon out */
       transition-property: filter, opacity;
@@ -538,10 +534,6 @@ tab-split-view-wrapper[dragtarget] {
 
   @media -moz-pref("browser.tabs.fadeOutExplicitlyUnloadedTabs") {
     &[pending][discarded] {
-      filter: grayscale(100%);
-      @media (prefers-color-scheme: dark) {
-        filter: grayscale(100%) invert();
-      }
       opacity: 0.5;
       /* Fade the favicon out */
       transition-property: filter, opacity;
@@ -615,7 +607,7 @@ tab-split-view-wrapper[dragtarget] {
   }
 
   #tabbrowser-tabs[orient="vertical"] & {
-    top: 7px;
+    top: -7px;
   }
 
   &[crashed] {
@@ -623,7 +615,7 @@ tab-split-view-wrapper[dragtarget] {
   }
 
   #tabbrowser-tabs[orient="vertical"]:not([expanded]) &:not([crashed]),
-  &[pinned]:not([crashed]) {
+  &:not([crashed]) {
     &[soundplaying] {
       list-style-image: url("chrome://browser/skin/tabbrowser/tab-audio-playing-small.svg");
     }
@@ -677,7 +669,7 @@ tab-split-view-wrapper[dragtarget] {
     }
   }
 
-  &[pinned]:is([soundplaying], [muted], [activemedia-blocked]),
+  &[zen-essential]:is([soundplaying], [muted], [activemedia-blocked]),
   &[crashed] {
     display: revert;
   }
@@ -837,7 +829,7 @@ tab-split-view-wrapper[dragtarget] {
       has not been added to root. There are certain scenarios when that attribute is temporarily
       removed from root such as when toggling the sidebar to expand with the toolbar button. */
     #tabbrowser-tabs[orient="horizontal"] &:not([pinned]):not([crashed]),
-    :root:not([sidebar-expand-on-hover]) #tabbrowser-tabs[orient="vertical"][expanded] &:not([pinned]):not([crashed]) {
+    :root:not([sidebar-expand-on-hover]) #tabbrowser-tabs[orient="vertical"][expanded] &:not([zen-essential]):not([crashed]) {
       &:is([soundplaying], [muted], [activemedia-blocked]) {
         display: flex;
       }
@@ -1151,7 +1143,6 @@ tab-split-view-wrapper[dragtarget] {
 .tabbrowser-tab:is([image], [pinned]) > .tab-stack > .tab-content[attention]:not([selected]),
 .tabbrowser-tab > .tab-stack > .tab-content[pinned][titlechanged]:not([selected]),
 #tabbrowser-tabs[orient="vertical"] .tabbrowser-tab > .tab-stack > .tab-content[titlechanged]:not([selected]) {
-  background-image: radial-gradient(circle, var(--tab-attention-dot-color), var(--tab-attention-dot-color) 2px, transparent 2px);
   background-position: center bottom 6.5px;
   background-size: 4px 4px;
   background-repeat: no-repeat;
@@ -1904,7 +1895,7 @@ tab-group {
 }
 
 #tabbrowser-tabs[orient="vertical"][expanded] {
-  tab-group > :is(.tab-group-label-container, .tabbrowser-tab),
+  tab-group > :is(.tab-group-label-container, .tab-group-container, .tabbrowser-tab),
   &[movingtab][movingtab-addToGroup]:not([movingtab-group], [movingtab-ungroup]) .tabbrowser-tab:is(:active, [multiselected]) {
     margin-inline-start: var(--space-medium);
   }
@@ -2504,7 +2495,7 @@ tab-group {
   }
 }
 
-#tabbrowser-arrowscrollbox[orient="vertical"] > #tabbrowser-arrowscrollbox-periphery > #tabs-newtab-button,
+#tabs-newtab-button,
 #vertical-tabs-newtab-button {
   appearance: none;
   min-height: var(--tab-min-height);
@@ -2524,7 +2515,7 @@ tab-group {
   }
 
   #tabbrowser-tabs[orient="vertical"]:not([expanded]) & > .toolbarbutton-text {
-    display: none;
+    display: flex;
   }
 
   &:hover {
@@ -2548,7 +2539,7 @@ tab-group {
  * flex container. #tabs-newtab-button is a child of the arrowscrollbox where
  * we don't want a gap (between tabs), so we have to add some margin.
  */
-#tabbrowser-arrowscrollbox[orient="vertical"] > #tabbrowser-arrowscrollbox-periphery > #tabs-newtab-button {
+#tabbrowser-arrowscrollbox[orient="vertical"] #tabbrowser-arrowscrollbox-periphery > #tabs-newtab-button {
   margin-block: var(--tab-margin-block);
 }
 
@@ -2740,7 +2731,6 @@ tab-group {
 
   &:not([expanded]) {
     .tabbrowser-tab[pinned] {
-      width: var(--tab-collapsed-width);
     }
 
     .tab-background {
@@ -2779,8 +2769,8 @@ tab-group {
       display: block;
       position: absolute;
       inset: auto;
-      top: -3px;
-      inset-inline-start: 2px;
+      top: -2px;
+      inset-inline-start: -2px;
 
       &:-moz-window-inactive {
         background-image:
@@ -2878,9 +2868,6 @@ tab-group {
   ~ #tabbrowser-tabs[orient="horizontal"]::before {
     display: flex;
     content: "";
-    border-inline-start: var(--tabstrip-inner-border);
-    padding-inline-start: 2px;
-    margin-inline-start: 2px;
   }
 }
 
@@ -2913,7 +2900,6 @@ toolbar:not(#TabsToolbar) #firefox-view-button {
   list-style-image: url(chrome://global/skin/icons/plus.svg);
 }
 
-#tabbrowser-tabs[hasadjacentnewtabbutton]:not([overflow]) ~ #new-tab-button,
 #tabbrowser-tabs[orient="horizontal"] > #vertical-tabs-newtab-button,
 #tabbrowser-tabs[orient="vertical"]:not([overflow]) > #vertical-tabs-newtab-button,
 #tabbrowser-arrowscrollbox[overflowing] > #tabbrowser-arrowscrollbox-periphery > #tabs-newtab-button,