diff --git a/browser/themes/linux/browser.css b/browser/themes/linux/browser.css
index a14abba4c209..5ab2496f6769 100644
--- a/browser/themes/linux/browser.css
+++ b/browser/themes/linux/browser.css
@@ -21,7 +21,6 @@
 
   --toolbarbutton-border-radius: 4px;
   --toolbarbutton-vertical-text-padding: calc(var(--toolbarbutton-inner-padding) - 1px);
-  --toolbarbutton-icon-fill-opacity: 1;
 
   --panel-separator-color: ThreeDShadow;
 
diff --git a/browser/themes/osx/browser.css b/browser/themes/osx/browser.css
index 17279ffcc684..378edbdb7920 100644
--- a/browser/themes/osx/browser.css
+++ b/browser/themes/osx/browser.css
@@ -14,7 +14,6 @@
 
   --toolbarbutton-vertical-text-padding: calc(var(--toolbarbutton-inner-padding) + 1px);
   --toolbarbutton-border-radius: 4px;
-  --toolbarbutton-icon-fill-opacity: 1;
 
   --panel-separator-color: hsla(210,4%,10%,.14);
 
diff --git a/browser/themes/shared/customizableui/panelUI.inc.css b/browser/themes/shared/customizableui/panelUI.inc.css
index d0ba93189b15..a2ddb1447cc2 100644
--- a/browser/themes/shared/customizableui/panelUI.inc.css
+++ b/browser/themes/shared/customizableui/panelUI.inc.css
@@ -216,7 +216,6 @@ toolbarseparator + .panel-subview-body,
   list-style-image: url(chrome://global/skin/icons/info.svg);
   -moz-context-properties: fill, fill-opacity;
   fill: currentColor;
-  fill-opacity: var(--toolbarbutton-icon-fill-opacity);
   padding: 4px;
 }
 
diff --git a/browser/themes/shared/notification-icons.inc.css b/browser/themes/shared/notification-icons.inc.css
index 37f7d030b3d6..43d30eec9aff 100644
--- a/browser/themes/shared/notification-icons.inc.css
+++ b/browser/themes/shared/notification-icons.inc.css
@@ -24,7 +24,6 @@
 .permission-popup-permission-icon {
   -moz-context-properties: fill, fill-opacity;
   fill: currentColor;
-  fill-opacity: var(--toolbarbutton-icon-fill-opacity);
 }
 
 .blocked-permission-icon:-moz-focusring,
diff --git a/browser/themes/shared/tabs.inc.css b/browser/themes/shared/tabs.inc.css
index 63f5ef491a66..5c497dfcc22f 100644
--- a/browser/themes/shared/tabs.inc.css
+++ b/browser/themes/shared/tabs.inc.css
@@ -446,7 +446,6 @@
 
 .tab-close-button {
   -moz-context-properties: fill, fill-opacity;
-  fill-opacity: var(--toolbarbutton-icon-fill-opacity);
   margin-inline-end: calc(var(--inline-tab-padding) / -2);
   width: 24px;
   height: 24px;
@@ -683,7 +682,6 @@
 #tabbrowser-arrowscrollbox::part(scrollbutton-up),
 #tabbrowser-arrowscrollbox::part(scrollbutton-down) {
   fill: var(--toolbarbutton-icon-fill);
-  fill-opacity: var(--toolbarbutton-icon-fill-opacity);
 }
 
 /* New tab button */
@@ -707,7 +705,6 @@
   display: block;
   -moz-context-properties: fill, fill-opacity, stroke;
   fill: currentColor;
-  fill-opacity: var(--toolbarbutton-icon-fill-opacity);
   stroke: transparent;
   /* "!important" is necessary to override the rule in toolbarbutton.css */
   margin: -7px 0 0 !important;
diff --git a/browser/themes/shared/toolbarbutton-icons.inc.css b/browser/themes/shared/toolbarbutton-icons.inc.css
index 5995a1f311d6..27fc0b42056c 100644
--- a/browser/themes/shared/toolbarbutton-icons.inc.css
+++ b/browser/themes/shared/toolbarbutton-icons.inc.css
@@ -7,7 +7,6 @@
 }
 
 :root:-moz-lwtheme {
-  --toolbarbutton-icon-fill-opacity: 1;
   --toolbarbutton-icon-fill-attention: var(--lwt-toolbarbutton-icon-fill-attention, rgb(0,97,224));
 }
 
@@ -21,7 +20,6 @@ toolbar[brighttext]:-moz-lwtheme {
   color: inherit !important;
   -moz-context-properties: fill, fill-opacity;
   fill: var(--toolbarbutton-icon-fill);
-  fill-opacity: var(--toolbarbutton-icon-fill-opacity);
 }
 
 .toolbarbutton-animatable-box {
@@ -420,7 +418,6 @@ toolbar[brighttext] #panic-button[open] {
 #save-to-pocket-button {
   list-style-image: url("chrome://browser/skin/pocket-outline.svg");
   fill: var(--toolbarbutton-icon-fill);
-  fill-opacity: var(--toolbarbutton-icon-fill-opacity);
 }
 
 #save-to-pocket-button[open="true"],
@@ -440,7 +437,6 @@ toolbar[brighttext] #panic-button[open] {
 toolbarbutton.bookmark-item {
   -moz-context-properties: fill, fill-opacity;
   fill: var(--toolbarbutton-icon-fill);
-  fill-opacity: var(--toolbarbutton-icon-fill-opacity);
 }
 
 .bookmark-item[container] {
diff --git a/browser/themes/shared/urlbar-searchbar.inc.css b/browser/themes/shared/urlbar-searchbar.inc.css
index 04a2494b707d..737bb936de69 100644
--- a/browser/themes/shared/urlbar-searchbar.inc.css
+++ b/browser/themes/shared/urlbar-searchbar.inc.css
@@ -176,7 +176,6 @@
   background-position: center;
   -moz-context-properties: fill, fill-opacity;
   fill: var(--toolbarbutton-icon-fill);
-  fill-opacity: var(--toolbarbutton-icon-fill-opacity);
 }
 
 #urlbar.searchButton > #urlbar-input-container[pageproxystate="invalid"] > #urlbar-search-button {
diff --git a/browser/themes/windows/browser-aero.css b/browser/themes/windows/browser-aero.css
index b1b3a772ae4c..c6c370d9f2ed 100644
--- a/browser/themes/windows/browser-aero.css
+++ b/browser/themes/windows/browser-aero.css
@@ -73,7 +73,6 @@
           }
           :root[tabsintitlebar][lwt-default-theme-in-dark-mode] #titlebar {
             --toolbarbutton-icon-fill: currentColor;
-            --toolbarbutton-icon-fill-opacity: .7;
           }
         }
         } /*** end !proton ***/
@@ -327,11 +326,6 @@
 }
 
 @media (-moz-windows-glass) {
-  /* Set to full fill-opacity to improve visibility of toolbar buttons on aero glass. */
-  #TabsToolbar:not(:-moz-lwtheme) {
-    --toolbarbutton-icon-fill-opacity: 1;
-  }
-
   #toolbar-menubar:not(:-moz-lwtheme) {
     text-shadow: 0 0 .5em white, 0 0 .5em white, 0 1px 0 rgba(255,255,255,.4);
   }
diff --git a/browser/themes/windows/browser.css b/browser/themes/windows/browser.css
index c53af069cb96..13006968c653 100644
--- a/browser/themes/windows/browser.css
+++ b/browser/themes/windows/browser.css
@@ -16,7 +16,6 @@
 
   --toolbarbutton-vertical-text-padding: calc(var(--toolbarbutton-inner-padding) - 1px);
   --toolbarbutton-border-radius: 4px;
-  --toolbarbutton-icon-fill-opacity: 1;
 
   --panel-separator-color: ThreeDLightShadow;
 
diff --git a/toolkit/components/printing/content/printPagination.css b/toolkit/components/printing/content/printPagination.css
index 621ed6deff6c..8eb84773f3fa 100644
--- a/toolkit/components/printing/content/printPagination.css
+++ b/toolkit/components/printing/content/printPagination.css
@@ -69,7 +69,6 @@
   background-size: 12px;
   -moz-context-properties: fill, fill-opacity;
   fill: var(--toolbarbutton-icon-fill);
-  fill-opacity: var(--toolbarbutton-icon-fill-opacity);
 }
 
 .toolbarButton:hover {