--- a/browser/themes/linux/browser.css
+++ b/browser/themes/linux/browser.css
@@ -68,17 +68,17 @@
   background-clip: padding-box;
   margin-top: -1px; /* Move up into the TabsToolbar for the inner highlight at the top of the nav-bar */
   /* Position the toolbar above the bottom of background tabs */
   position: relative;
   z-index: 1;
 }
 
 #nav-bar {
-  background-image: linear-gradient(@toolbarHighlight@, rgba(255,255,255,0));
+  background-image: linear-gradient(@toolbarHighlight@, transparent);
   box-shadow: 0 1px 0 @toolbarHighlight@ inset;
   padding-top: 2px;
   padding-bottom: 2px;
 }
 
 #nav-bar-overflow-button {
   -moz-image-region: rect(-5px, 12px, 11px, -4px);
 }
@@ -1535,58 +1535,58 @@ richlistitem[type~="action"][actiontype=
   list-style-image: url("chrome://browser/skin/reload-stop-go.png");
 }
 
 #urlbar-reload-button {
   -moz-image-region: rect(0, 14px, 14px, 0);
 }
 
 #urlbar-reload-button:not([disabled]):hover {
-  background-image: radial-gradient(circle closest-side, hsla(200,100%,70%,.2), hsla(200,100%,70%,0));
+  background-image: radial-gradient(circle closest-side, hsla(200,100%,70%,.2), transparent);
   -moz-image-region: rect(14px, 14px, 28px, 0);
 }
 
 #urlbar-reload-button:not([disabled]):hover:active {
-  background-image: radial-gradient(circle closest-side, hsla(200,100%,60%,.1), hsla(200,100%,60%,0));
+  background-image: radial-gradient(circle closest-side, hsla(200,100%,60%,.1), transparent);
   -moz-image-region: rect(28px, 14px, 42px, 0);
 }
 
 #urlbar-reload-button:-moz-locale-dir(rtl) > .toolbarbutton-icon {
   transform: scaleX(-1);
 }
 
 #urlbar-go-button {
   -moz-image-region: rect(0, 42px, 14px, 28px);
 }
 
 #urlbar-go-button:hover {
-  background-image: radial-gradient(circle closest-side, hsla(110,70%,50%,.2), hsla(110,70%,50%,0));
+  background-image: radial-gradient(circle closest-side, hsla(110,70%,50%,.2), transparent);
   -moz-image-region: rect(14px, 42px, 28px, 28px);
 }
 
 #urlbar-go-button:hover:active {
-  background-image: radial-gradient(circle closest-side, hsla(110,70%,50%,.1), hsla(110,70%,50%,0));
+  background-image: radial-gradient(circle closest-side, hsla(110,70%,50%,.1), transparent);
   -moz-image-region: rect(28px, 42px, 42px, 28px);
 }
 
 #urlbar-go-button:-moz-locale-dir(rtl) > .toolbarbutton-icon {
   transform: scaleX(-1);
 }
 
 #urlbar-stop-button {
   -moz-image-region: rect(0, 28px, 14px, 14px);
 }
 
 #urlbar-stop-button:not([disabled]):hover {
-  background-image: radial-gradient(circle closest-side, hsla(5,100%,75%,.3), hsla(5,100%,75%,0));
+  background-image: radial-gradient(circle closest-side, hsla(5,100%,75%,.3), transparent);
   -moz-image-region: rect(14px, 28px, 28px, 14px);
 }
 
 #urlbar-stop-button:hover:active {
-  background-image: radial-gradient(circle closest-side, hsla(5,100%,75%,.1), hsla(5,100%,75%,0));
+  background-image: radial-gradient(circle closest-side, hsla(5,100%,75%,.1), transparent);
   -moz-image-region: rect(28px, 28px, 42px, 14px);
 }
 
 /* Popup blocker button */
 #page-report-button {
   list-style-image: url("chrome://browser/skin/Info.png");
 }
 
@@ -2118,40 +2118,40 @@ toolbarbutton.chevron > .toolbarbutton-i
 .social-panel > .panel-arrowcontainer > .panel-arrowcontent {
   padding: 0;
 }
 
 %include ../shared/social/chat.inc.css
 
 .chat-titlebar {
   background-color: #d9d9d9;
-  background-image: linear-gradient(@toolbarHighlight@, rgba(255,255,255,0));
+  background-image: linear-gradient(@toolbarHighlight@, transparent);
 }
 
 .chat-titlebar[selected] {
   background-color: #f0f0f0;
 }
 
 .chatbar-button {
   -moz-appearance: none;
   background-color: #d9d9d9;
-  background-image: linear-gradient(@toolbarHighlight@, rgba(255,255,255,0));
+  background-image: linear-gradient(@toolbarHighlight@, transparent);
 }
 
 .chatbar-button > .toolbarbutton-icon {
   -moz-margin-end: 0;
 }
 
 .chatbar-button:hover,
 .chatbar-button[open="true"] {
   background-color: #f0f0f0;
 }
 
 .chatbar-button[activity] {
-  background-image: radial-gradient(circle farthest-corner at center 3px, rgb(233,242,252) 3%, rgba(172,206,255,0.75) 40%, rgba(87,151,201,0.5) 80%, rgba(87,151,201,0));
+  background-image: radial-gradient(circle farthest-corner at center 3px, rgb(233,242,252) 3%, rgba(172,206,255,0.75) 40%, rgba(87,151,201,0.5) 80%, transparent);
 }
 
 chatbox {
   border-top-left-radius: 2.5px;
   border-top-right-radius: 2.5px;
 }
 
 /* Customization mode */

--- a/browser/themes/linux/downloads/downloads.css
+++ b/browser/themes/linux/downloads/downloads.css
@@ -164,17 +164,17 @@ richlistitem[type="download"]:last-child
 
 /*** Highlighted list items ***/
 
 #downloadsPanel:not([keyfocus]) > #downloadsListBox > richlistitem[type="download"][state="1"][exists]:hover {
   border-radius: 3px;
   border-top: 1px solid hsla(0,0%,100%,.3);
   border-bottom: 1px solid hsla(0,0%,0%,.2);
   background-color: Highlight;
-  background-image: linear-gradient(hsla(0,0%,100%,.1), hsla(0,0%,100%,0));
+  background-image: linear-gradient(hsla(0,0%,100%,.1), transparent);
   color: HighlightText;
   cursor: pointer;
 }
 
 /*** Button icons ***/
 
 .downloadButton.downloadCancel {
   -moz-image-region: rect(0px, 16px, 16px, 0px);

--- a/browser/themes/osx/browser.css
+++ b/browser/themes/osx/browser.css
@@ -20,17 +20,17 @@
   --space-above-tabbar: 9px;
 
   --tabs-toolbar-color: #333;
   --toolbarbutton-hover-background: hsla(0,0%,100%,.1) linear-gradient(hsla(0,0%,100%,.3), hsla(0,0%,100%,.1)) padding-box;
   --toolbarbutton-hover-bordercolor: hsla(0,0%,0%,.2);
   --toolbarbutton-hover-boxshadow: 0 1px 0 hsla(0,0%,100%,.5),
                                    0 1px 0 hsla(0,0%,100%,.5) inset;
 
-  --toolbarbutton-active-background: hsla(0,0%,0%,.02) linear-gradient(hsla(0,0%,0%,.12), hsla(0,0%,0%,0)) border-box;
+  --toolbarbutton-active-background: hsla(0,0%,0%,.02) linear-gradient(hsla(0,0%,0%,.12), transparent) border-box;
   --toolbarbutton-active-boxshadow: 0 1px 0 hsla(0,0%,100%,.5),
                                     0 1px 0 hsla(0,0%,0%,.05) inset,
                                     0 1px 1px hsla(0,0%,0%,.2) inset;
 
   --toolbarbutton-checkedhover-backgroundcolor: hsla(0,0%,0%,.09);
 
   --toolbarbutton-combined-boxshadow: 0 0 0 1px hsla(0,0%,100%,.15);
   --toolbarbutton-combined-backgroundimage: linear-gradient(hsla(0,0%,0%,.15) 0, hsla(0,0%,0%,.15) 18px);
@@ -2123,17 +2123,17 @@ toolbarbutton[sdk-button="true"][cui-are
   padding: 0 3px;
   list-style-image: var(--urlbar-dropmarker-url);
   -moz-image-region: var(--urlbar-dropmarker-region);
 }
 
 .urlbar-history-dropmarker[open="true"],
 .urlbar-history-dropmarker:hover:active {
   -moz-image-region: var(--urlbar-dropmarker-active-region);
-  background-image: radial-gradient(circle closest-side, hsla(205,100%,70%,.3), hsla(205,100%,70%,0));
+  background-image: radial-gradient(circle closest-side, hsla(205,100%,70%,.3), transparent);
 }
 
 @media (min-resolution: 2dppx) {
   .urlbar-history-dropmarker {
     list-style-image: var(--urlbar-dropmarker-2x-url);
     -moz-image-region: var(--urlbar-dropmarker-2x-region);
   }
 
@@ -2152,17 +2152,17 @@ toolbarbutton[sdk-button="true"][cui-are
 }
 
 .urlbar-icon {
   padding: 0 3px;
 }
 
 .urlbar-icon[open="true"],
 .urlbar-icon:hover:active {
-  background-image: radial-gradient(circle closest-side, hsla(205,100%,70%,.3), hsla(205,100%,70%,0));
+  background-image: radial-gradient(circle closest-side, hsla(205,100%,70%,.3), transparent);
 }
 
 #urlbar-search-splitter {
   min-width: 8px;
   width: 8px;
   background-image: none;
   margin: 0 -4px;
   position: relative;
@@ -2368,17 +2368,17 @@ richlistitem[type~="action"][actiontype=
   margin: 0;
   -moz-padding-start: 2px;
   -moz-padding-end: 1px;
   background-origin: border-box;
   list-style-image: url("chrome://browser/skin/reload-stop-go.png");
 }
 
 #urlbar > toolbarbutton:not([disabled]):hover:active {
-  background-image: radial-gradient(circle closest-side, hsla(205,100%,70%,.3), hsla(205,100%,70%,0));
+  background-image: radial-gradient(circle closest-side, hsla(205,100%,70%,.3), transparent);
 }
 
 #urlbar-go-button {
   -moz-image-region: rect(0, 42px, 14px, 28px);
 }
 
 #urlbar-go-button:hover:active {
   -moz-image-region: rect(14px, 42px, 28px, 28px);
@@ -4518,26 +4518,26 @@ menulist.translate-infobar-element > .me
 }
 
 /* === end of social toolbar provider menu === */
 
 %include ../shared/social/chat.inc.css
 
 .chat-titlebar {
   background-color: #d9d9d9;
-  background-image: linear-gradient(rgba(255,255,255,.43), rgba(255,255,255,0));
+  background-image: linear-gradient(rgba(255,255,255,.43), transparent);
 }
 
 .chat-titlebar[selected] {
   background-color: #f0f0f0;
 }
 
 .chatbar-button {
   background-color: #d9d9d9;
-  background-image: linear-gradient(rgba(255,255,255,.43), rgba(255,255,255,0));
+  background-image: linear-gradient(rgba(255,255,255,.43), transparent);
   border-top-left-radius: @toolbarbuttonCornerRadius@;
   border-top-right-radius: @toolbarbuttonCornerRadius@;
 }
 
 .chatbar-button:hover,
 .chatbar-button[open="true"] {
   background-color: #f0f0f0;
 }

--- a/browser/themes/osx/customizableui/panelUIOverlay.css
+++ b/browser/themes/osx/customizableui/panelUIOverlay.css
@@ -7,24 +7,24 @@
 .panel-subviews {
   background-color: hsla(0,0%,100%,.97);
 }
 
 @media (min-resolution: 2dppx) {
   #PanelUI-help[panel-multiview-anchor="true"]::after,
   toolbarbutton[panel-multiview-anchor="true"] {
     background-image: url(chrome://browser/skin/customizableui/subView-arrow-back-inverted@2x.png),
-                      linear-gradient(rgba(255,255,255,0.3), rgba(255,255,255,0));
+                      linear-gradient(rgba(255,255,255,0.3), transparent);
     background-size: 16px, auto;
   }
 
   #PanelUI-help[panel-multiview-anchor="true"]:-moz-locale-dir(rtl)::after,
   toolbarbutton[panel-multiview-anchor="true"]:-moz-locale-dir(rtl) {
     background-image: url(chrome://browser/skin/customizableui/subView-arrow-back-inverted-rtl@2x.png),
-                      linear-gradient(rgba(255,255,255,0.3), rgba(255,255,255,0));
+                      linear-gradient(rgba(255,255,255,0.3), transparent);
   }
 
   #PanelUI-fxa-status {
     list-style-image: url(chrome://browser/skin/sync-horizontalbar@2x.png);
   }
 
   #PanelUI-fxa-status[status="active"] {
     list-style-image: url(chrome://browser/skin/syncProgress-horizontalbar@2x.png);

--- a/browser/themes/shared/customizableui/customizeMode.inc.css
+++ b/browser/themes/shared/customizableui/customizeMode.inc.css
@@ -347,32 +347,32 @@ toolbarpaletteitem[place="toolbar"] {
 }
 
 #customization-lwtheme-menu-recommended {
   border-top: 1px solid hsla(210,4%,10%,.05);
   border-bottom: 1px solid hsla(210,4%,10%,.05);
 }
 
 #customization-lwtheme-menu-footer {
-  background: linear-gradient(hsla(210,4%,10%,.05) 60%, hsla(210,4%,10%,0)) border-box;
+  background: linear-gradient(hsla(210,4%,10%,.05) 60%, transparent) border-box;
   border-top: 1px solid hsla(210,4%,10%,.05);
   margin-bottom: -10px;
 }
 
 .customization-lwtheme-menu-footeritem {
   -moz-appearance: none;
   -moz-box-flex: 1;
   color: hsl(0,0%,50%);
   border-style: none;
   padding: 10px;
   margin-left: 0;
   margin-right: 0;
 }
 
 .customization-lwtheme-menu-footeritem:hover {
-  background: linear-gradient(hsla(210,4%,10%,.08) 40%, hsla(210,4%,10%,0)) padding-box;
+  background: linear-gradient(hsla(210,4%,10%,.08) 40%, transparent) padding-box;
 }
 
 .customization-lwtheme-menu-footeritem:first-child {
   -moz-border-end: 1px solid hsla(210,4%,10%,.15);
 }
 
 %include customizeTip.inc.css

--- a/browser/themes/shared/customizableui/panelUIOverlay.inc.css
+++ b/browser/themes/shared/customizableui/panelUIOverlay.inc.css
@@ -82,19 +82,19 @@
   from { margin-top: -.5em; } to { margin-top: calc(64px - .5em); }
 }
 
 @keyframes whimsyRotate {
   to { transform: perspective(5000px) rotateY(360deg); }
 }
 
 #PanelUI-button {
-  background-image: linear-gradient(to bottom, hsla(0,0%,100%,0), hsla(0,0%,100%,.3) 30%, hsla(0,0%,100%,.3) 70%, hsla(0,0%,100%,0)),
-                    linear-gradient(to bottom, hsla(210,54%,20%,0), hsla(210,54%,20%,.3) 30%, hsla(210,54%,20%,.3) 70%, hsla(210,54%,20%,0)),
-                    linear-gradient(to bottom, hsla(0,0%,100%,0), hsla(0,0%,100%,.3) 30%, hsla(0,0%,100%,.3) 70%, hsla(0,0%,100%,0));
+  background-image: linear-gradient(to bottom, transparent, hsla(0,0%,100%,.3) 30%, hsla(0,0%,100%,.3) 70%, transparent),
+                    linear-gradient(to bottom, transparent, hsla(210,54%,20%,.3) 30%, hsla(210,54%,20%,.3) 70%, transparent),
+                    linear-gradient(to bottom, transparent, hsla(0,0%,100%,.3) 30%, hsla(0,0%,100%,.3) 70%, transparent);
   background-size: 1px calc(100% - 1px), 1px calc(100% - 1px), 1px  calc(100% - 1px) !important;
   background-position: 0px 0px, 1px 0px, 2px 0px;
   background-repeat: no-repeat;
 }
 
 #PanelUI-button:-moz-locale-dir(rtl) {
   background-position: 100% 0, calc(100% - 1px) 0, calc(100% - 2px) 0;
 }
@@ -869,49 +869,49 @@ toolbarbutton[panel-multiview-anchor="tr
   background-color: Highlight;
 }
 
 #PanelUI-help[panel-multiview-anchor="true"] + toolbarseparator {
   display: none;
 }
 
 #PanelUI-help[panel-multiview-anchor="true"] {
-  background-image: linear-gradient(rgba(255,255,255,0.3), rgba(255,255,255,0));
+  background-image: linear-gradient(rgba(255,255,255,0.3), transparent);
   background-position: 0;
 }
 
 #PanelUI-help[panel-multiview-anchor="true"]::after {
   content: "";
   position: absolute;
   top: 0;
   height: 100%;
   width: @exitSubviewGutterWidth@;
   background-image: url(chrome://browser/skin/customizableui/subView-arrow-back-inverted.png),
-                    linear-gradient(rgba(255,255,255,0.3), rgba(255,255,255,0));
+                    linear-gradient(rgba(255,255,255,0.3), transparent);
   background-repeat: no-repeat;
   background-color: Highlight;
   background-position: left 10px center, 0;
 }
 
 #PanelUI-help[panel-multiview-anchor="true"]:-moz-locale-dir(rtl)::after {
   background-image: url(chrome://browser/skin/customizableui/subView-arrow-back-inverted-rtl.png),
-                    linear-gradient(rgba(255,255,255,0.3), rgba(255,255,255,0));
+                    linear-gradient(rgba(255,255,255,0.3), transparent);
   background-position: right 10px center, 0;
 }
 
 toolbarbutton[panel-multiview-anchor="true"] {
   background-image: url(chrome://browser/skin/customizableui/subView-arrow-back-inverted.png),
-                    linear-gradient(rgba(255,255,255,0.3), rgba(255,255,255,0));
+                    linear-gradient(rgba(255,255,255,0.3), transparent);
   background-position: right calc(@menuPanelButtonWidth@ / 2 - @exitSubviewGutterWidth@ + 2px) center;
   background-repeat: no-repeat, repeat;
 }
 
 toolbarbutton[panel-multiview-anchor="true"]:-moz-locale-dir(rtl) {
   background-image: url(chrome://browser/skin/customizableui/subView-arrow-back-inverted-rtl.png),
-                    linear-gradient(rgba(255,255,255,0.3), rgba(255,255,255,0));
+                    linear-gradient(rgba(255,255,255,0.3), transparent);
   background-position: left calc(@menuPanelButtonWidth@ / 2 - @exitSubviewGutterWidth@ + 2px) center;
 }
 
 toolbarpaletteitem[place="palette"] > .toolbarbutton-1 > .toolbarbutton-menubutton-dropmarker,
 #bookmarks-menu-button[cui-areatype="menu-panel"] > .toolbarbutton-menubutton-dropmarker {
   display: none;
 }
 
@@ -1000,19 +1000,19 @@ toolbarpaletteitem[haswideitem][place="p
   border-top-left-radius: 0;
   border-bottom-left-radius: 0;
 }
 
 .toolbaritem-combined-buttons > separator {
   -moz-appearance: none;
   width: 3px;
   -moz-box-align: stretch;
-  background-image: linear-gradient(to bottom, hsla(0,0%,100%,0), hsla(0,0%,100%,.3) 40%, hsla(0,0%,100%,.3) 60%, hsla(0,0%,100%,0)),
-                    linear-gradient(to bottom, hsla(210,54%,20%,0), hsla(210,54%,20%,.15) 40%, hsla(210,54%,20%,.15) 60%, hsla(210,54%,20%,0)),
-                    linear-gradient(to bottom, hsla(0,0%,100%,0), hsla(0,0%,100%,.3) 40%, hsla(0,0%,100%,.3) 60%, hsla(0,0%,100%,0));
+  background-image: linear-gradient(to bottom, transparent, hsla(0,0%,100%,.3) 40%, hsla(0,0%,100%,.3) 60%, transparent),
+                    linear-gradient(to bottom, transparent, hsla(210,54%,20%,.15) 40%, hsla(210,54%,20%,.15) 60%, transparent),
+                    linear-gradient(to bottom, transparent, hsla(0,0%,100%,.3) 40%, hsla(0,0%,100%,.3) 60%, transparent);
   background-size: 1px, 1px, 1px;
   background-position: 0 0, 1px 0, 2px 0;
   background-repeat: no-repeat;
 }
 
 .toolbaritem-combined-buttons@inAnyPanel@ > separator {
   margin: .5em 0;
   width: 1px;

--- a/browser/themes/shared/devtools/app-manager/device.css
+++ b/browser/themes/shared/devtools/app-manager/device.css
@@ -298,17 +298,17 @@ button {
   float: right;
   padding: 0 5px;
 }
 
 /*****************     HEADER      *****************/
 
 header {
   padding-top: 140px;
-  background-image: linear-gradient(to bottom, rgba(0,0,0,0), rgba(0,0,0,0.7));
+  background-image: linear-gradient(to bottom, transparent, rgba(0,0,0,0.7));
   color: #FFF;
   text-shadow: 0 1px 2px rgba(0,0,0,0.8);
   padding: 10px;
 }
 
 
 
 /*****************      APPS & BROWSER TABS      *****************/

--- a/browser/themes/shared/devtools/toolbars.inc.css
+++ b/browser/themes/shared/devtools/toolbars.inc.css
@@ -519,19 +519,19 @@
 #toolbox-dock-bottom:hover,
 #toolbox-dock-side:hover {
   opacity: 1;
 }
 
 .devtools-separator {
   margin: 0 2px;
   width: 2px;
-  background-image: linear-gradient(hsla(204,45%,98%,0), hsla(204,45%,98%,.1), hsla(204,45%,98%,0)),
-                    linear-gradient(hsla(206,37%,4%,0), hsla(206,37%,4%,.6), hsla(206,37%,4%,0)),
-                    linear-gradient(hsla(204,45%,98%,0), hsla(204,45%,98%,.1), hsla(204,45%,98%,0));
+  background-image: linear-gradient(transparent, hsla(204,45%,98%,.1), transparent),
+                    linear-gradient(transparent, hsla(206,37%,4%,.6), transparent),
+                    linear-gradient(transparent, hsla(204,45%,98%,.1), transparent);
   background-size: 1px 100%;
   background-repeat: no-repeat;
   background-position: 0, 1px, 2px;
 }
 
 #toolbox-buttons:empty + .devtools-separator,
 .devtools-separator[invisible] {
   visibility: hidden;

--- a/browser/themes/shared/social/chat.inc.css
+++ b/browser/themes/shared/social/chat.inc.css
@@ -121,17 +121,17 @@ chatbar > chatbox > .chat-titlebar > .ch
   margin-right: 2px;
 }
 
 .chat-titlebar[minimized="true"] {
   border-bottom: none;
 }
 
 .chat-titlebar[activity] {
-  background-image: radial-gradient(ellipse closest-side at center, rgb(255,255,255), rgba(255,255,255,0));
+  background-image: radial-gradient(ellipse closest-side at center, rgb(255,255,255), transparent);
   background-repeat: no-repeat;
   background-size: 100% 20px;
   background-position: 0 -10px;
 }
 
 chatbox[dark=true] > .chat-titlebar,
 chatbox[dark=true] > .chat-titlebar[selected] {
   border-bottom: none;

--- a/browser/themes/shared/tabs.inc.css
+++ b/browser/themes/shared/tabs.inc.css
@@ -289,17 +289,17 @@
 
 /* Pinned tab separators need position: absolute when positioned (during overflow). */
 #tabbrowser-tabs[positionpinnedtabs] > .tabbrowser-tab[pinned]::before {
   height: 100%;
   position: absolute;
 }
 
 .tabbrowser-tab[pinned][titlechanged]:not([selected="true"]) > .tab-stack > .tab-content {
-  background-image: radial-gradient(farthest-corner at center bottom, rgb(255,255,255) 3%, rgba(186,221,251,0.75) 20%, rgba(127,179,255,0.25) 40%, rgba(127,179,255,0) 70%);
+  background-image: radial-gradient(farthest-corner at center bottom, rgb(255,255,255) 3%, rgba(186,221,251,0.75) 20%, rgba(127,179,255,0.25) 40%, transparent 70%);
   background-position: center bottom @tabToolbarNavbarOverlap@;
   background-repeat: no-repeat;
   background-size: 85% 100%;
 }
 
 /* Background tab separators (3px wide).
    Also show separators beside the selected tab when dragging it. */
 #tabbrowser-tabs[movingtab] > .tabbrowser-tab[beforeselected]:not([last-visible-tab])::after,

--- a/browser/themes/windows/browser.css
+++ b/browser/themes/windows/browser.css
@@ -282,17 +282,17 @@
   /* Position the toolbar above the bottom of background tabs */
   position: relative;
   z-index: 1;
 }
 
 #nav-bar {
   border-top: 1px solid @toolbarShadowColor@ !important;
   background-clip: padding-box;
-  background-image: linear-gradient(@toolbarHighlight@, rgba(255,255,255,0));
+  background-image: linear-gradient(@toolbarHighlight@, transparent);
   box-shadow: 0 1px 0 @toolbarHighlight@ inset;
 }
 
 %ifdef WINDOWS_AERO
 @media not all and (-moz-windows-compositor) {
 %endif
   #TabsToolbar[collapsed="true"] + #nav-bar {
     border-top-style: none !important;
@@ -872,17 +872,17 @@ toolbarbutton[sdk-button="true"][cui-are
   margin-bottom: @tabToolbarNavbarOverlap@;
 }
 
 #TabsToolbar .toolbarbutton-1:not([disabled=true]):hover,
 #TabsToolbar .toolbarbutton-1[open],
 #TabsToolbar .toolbarbutton-1 > .toolbarbutton-menubutton-button:not([disabled=true]):hover,
 .tabbrowser-arrowscrollbox > .scrollbutton-up:not([disabled=true]):hover,
 .tabbrowser-arrowscrollbox > .scrollbutton-down:not([disabled=true]):hover {
-  background-image: linear-gradient(rgba(255,255,255,0), rgba(255,255,255,.5)),
+  background-image: linear-gradient(transparent, rgba(255,255,255,.5)),
                     linear-gradient(transparent, rgba(0,0,0,.25) 30%),
                     linear-gradient(transparent, rgba(0,0,0,.25) 30%);
   background-position: 1px -1px, 0 -1px, 100% -1px;
   background-size: calc(100% - 2px) 100%, 1px 100%, 1px 100%;
   background-repeat: no-repeat;
 }
 
 /* unified back/forward button */
@@ -1218,22 +1218,22 @@ html|*.urlbar-input:-moz-lwtheme::-moz-p
 }
 
 .searchbar-engine-button,
 .search-go-container {
   padding: 2px 2px;
 }
 
 .urlbar-icon:hover {
-  background-image: radial-gradient(circle closest-side, hsla(200,100%,70%,.3), hsla(200,100%,70%,0));
+  background-image: radial-gradient(circle closest-side, hsla(200,100%,70%,.3), transparent);
 }
 
 .urlbar-icon[open="true"],
 .urlbar-icon:hover:active {
-  background-image: radial-gradient(circle closest-side, hsla(200,100%,70%,.1), hsla(200,100%,70%,0));
+  background-image: radial-gradient(circle closest-side, hsla(200,100%,70%,.1), transparent);
 }
 
 #urlbar-search-splitter {
   min-width: 6px;
   -moz-margin-start: -3px;
   border: none;
   background: transparent;
 }
@@ -1338,23 +1338,23 @@ html|*.urlbar-input:-moz-lwtheme::-moz-p
   background-color: transparent;
   border: none;
   width: auto;
   list-style-image: url("chrome://browser/skin/urlbar-history-dropmarker.png");
   -moz-image-region: rect(0px, 11px, 14px, 0px);
 }
 
 .urlbar-history-dropmarker:hover {
-  background-image: radial-gradient(circle closest-side, hsla(205,100%,70%,.3), hsla(205,100%,70%,0));
+  background-image: radial-gradient(circle closest-side, hsla(205,100%,70%,.3), transparent);
   -moz-image-region: rect(0px, 22px, 14px, 11px);
 }
 
 .urlbar-history-dropmarker:hover:active,
 .urlbar-history-dropmarker[open="true"] {
-  background-image: radial-gradient(circle closest-side, hsla(205,100%,70%,.1), hsla(205,100%,70%,0));
+  background-image: radial-gradient(circle closest-side, hsla(205,100%,70%,.1), transparent);
   -moz-image-region: rect(0px, 33px, 14px, 22px);
 }
 
 /* page proxy icon */
 
 %include ../shared/identity-block.inc.css
 
 #page-proxy-favicon {
@@ -1490,58 +1490,58 @@ richlistitem[type~="action"][actiontype=
   list-style-image: url("chrome://browser/skin/reload-stop-go.png");
 }
 
 #urlbar-reload-button {
   -moz-image-region: rect(0, 14px, 14px, 0);
 }
 
 #urlbar-reload-button:not([disabled]):hover {
-  background-image: radial-gradient(circle closest-side, hsla(200,100%,70%,.2), hsla(200,100%,70%,0));
+  background-image: radial-gradient(circle closest-side, hsla(200,100%,70%,.2), transparent);
   -moz-image-region: rect(14px, 14px, 28px, 0);
 }
 
 #urlbar-reload-button:not([disabled]):hover:active {
-  background-image: radial-gradient(circle closest-side, hsla(200,100%,60%,.1), hsla(200,100%,60%,0));
+  background-image: radial-gradient(circle closest-side, hsla(200,100%,60%,.1), transparent);
   -moz-image-region: rect(28px, 14px, 42px, 0);
 }
 
 #urlbar-reload-button:-moz-locale-dir(rtl) > .toolbarbutton-icon {
   transform: scaleX(-1);
 }
 
 #urlbar-go-button {
   -moz-image-region: rect(0, 42px, 14px, 28px);
 }
 
 #urlbar-go-button:hover {
-  background-image: radial-gradient(circle closest-side, hsla(110,70%,50%,.2), hsla(110,70%,50%,0));
+  background-image: radial-gradient(circle closest-side, hsla(110,70%,50%,.2), transparent);
   -moz-image-region: rect(14px, 42px, 28px, 28px);
 }
 
 #urlbar-go-button:hover:active {
-  background-image: radial-gradient(circle closest-side, hsla(110,70%,50%,.1), hsla(110,70%,50%,0));
+  background-image: radial-gradient(circle closest-side, hsla(110,70%,50%,.1), transparent);
   -moz-image-region: rect(28px, 42px, 42px, 28px);
 }
 
 #urlbar-go-button:-moz-locale-dir(rtl) > .toolbarbutton-icon {
   transform: scaleX(-1);
 }
 
 #urlbar-stop-button {
   -moz-image-region: rect(0, 28px, 14px, 14px);
 }
 
 #urlbar-stop-button:not([disabled]):hover {
-  background-image: radial-gradient(circle closest-side, hsla(5,100%,75%,.3), hsla(5,100%,75%,0));
+  background-image: radial-gradient(circle closest-side, hsla(5,100%,75%,.3), transparent);
   -moz-image-region: rect(14px, 28px, 28px, 14px);
 }
 
 #urlbar-stop-button:hover:active {
-  background-image: radial-gradient(circle closest-side, hsla(5,100%,75%,.1), hsla(5,100%,75%,0));
+  background-image: radial-gradient(circle closest-side, hsla(5,100%,75%,.1), transparent);
   -moz-image-region: rect(28px, 28px, 42px, 14px);
 }
 
 /* popup blocker button */
 
 #page-report-button {
   list-style-image: url("chrome://browser/skin/urlbar-popup-blocked.png");
   -moz-image-region: rect(0, 16px, 16px, 0);
@@ -2767,27 +2767,27 @@ toolbarpaletteitem[place="palette"] > #s
 .social-panel-frame {
   border-radius: inherit;
 }
 
 %include ../shared/social/chat.inc.css
 
 .chat-titlebar {
   background-color: #c4cfde;
-  background-image: linear-gradient(rgba(255,255,255,.5), rgba(255,255,255,0));
+  background-image: linear-gradient(rgba(255,255,255,.5), transparent);
 }
 
 .chat-titlebar[selected] {
   background-color: #dae3f0;
 }
 
 .chatbar-button {
   -moz-appearance: none;
   background-color: #c4cfde;
-  background-image: linear-gradient(rgba(255,255,255,.5), rgba(255,255,255,0));
+  background-image: linear-gradient(rgba(255,255,255,.5), transparent);
 }
 
 .chatbar-button > .toolbarbutton-icon {
   -moz-margin-end: 0;
 }
 
 .chatbar-button:hover,
 .chatbar-button[open="true"] {

--- a/browser/themes/windows/downloads/allDownloadsViewOverlay-aero.css
+++ b/browser/themes/windows/downloads/allDownloadsViewOverlay-aero.css
@@ -16,20 +16,20 @@
 
   This styling should be kept in sync with the style from the above file.
   */
   #downloadsRichListBox > richlistitem.download[selected] {
     color: inherit;
     background-color: transparent;
     /* four gradients for the bevel highlights on each edge, one for blue background */
     background-image:
-      linear-gradient(to bottom, rgba(255,255,255,0.9) 3px, rgba(255,255,255,0) 3px),
-      linear-gradient(to right, rgba(255,255,255,0.5) 3px, rgba(255,255,255,0) 3px),
-      linear-gradient(to left, rgba(255,255,255,0.5) 3px, rgba(255,255,255,0) 3px),
-      linear-gradient(to top, rgba(255,255,255,0.4) 3px, rgba(255,255,255,0) 3px),
+      linear-gradient(to bottom, rgba(255,255,255,0.9) 3px, transparent 3px),
+      linear-gradient(to right, rgba(255,255,255,0.5) 3px, transparent 3px),
+      linear-gradient(to left, rgba(255,255,255,0.5) 3px, transparent 3px),
+      linear-gradient(to top, rgba(255,255,255,0.4) 3px, transparent 3px),
       linear-gradient(to bottom, rgba(163,196,247,0.3), rgba(122,180,246,0.3));
     background-clip: content-box;
     border-radius: 6px;
     outline: 1px solid rgb(124,163,206);
     -moz-outline-radius: 3px;
     outline-offset: -2px;
   }
 }

--- a/browser/themes/windows/places/organizer-aero.css
+++ b/browser/themes/windows/places/organizer-aero.css
@@ -27,17 +27,17 @@
 }
 
 @media (-moz-windows-compositor) {
   #placesToolbox {
     border-top: none;
   }
 
   #placesToolbar {
-    background-image: linear-gradient(@toolbarHighlight@, rgba(255,255,255,0));
+    background-image: linear-gradient(@toolbarHighlight@, transparent);
   }
 }
 
 @media (-moz-windows-default-theme) {
   #placesView > splitter {
     border: 0;
     -moz-border-end: 1px solid #A9B7C9;
     min-width: 0;

--- a/toolkit/themes/osx/global/tabprompts.css
+++ b/toolkit/themes/osx/global/tabprompts.css
@@ -31,17 +31,17 @@ tabmodalprompt {
 button {
   -moz-appearance: none;
   padding: 2px 0;
   margin: 0;
   -moz-margin-start: 8px;
   border-radius: 2px;
   color: black !important;
   background-color: hsl(0,0%,90%);
-  background-image: linear-gradient(hsla(0,0%,100%,.7), hsla(0,0%,100%,0));
+  background-image: linear-gradient(hsla(0,0%,100%,.7), transparent);
   background-clip: padding-box;
   border: 1px solid;
   border-color: hsl(0,0%,65%) hsl(0,0%,60%) hsl(0,0%,50%);
   box-shadow: 0 1px 0 hsla(0,0%,100%,.9) inset,
               0 1px 2px hsla(0,0%,0%,.1);
 }
 
 
@@ -49,17 +49,17 @@ button[default=true] {
   background-color: hsl(0,0%,79%);
 }
 
 button:hover {
   background-color: hsl(0,0%,96%);
 }
 
 button:hover:active {
-  background-image: linear-gradient(hsla(0,0%,100%,.2), hsla(0,0%,100%,0));
+  background-image: linear-gradient(hsla(0,0%,100%,.2), transparent);
   background-color: hsl(0,0%,70%);
   box-shadow: 0 1px 0 hsla(0,0%,100%,.2) inset,
               0 1px 3px hsla(0,0%,0%,.2);
 }
 
 button:focus {
   box-shadow: 0 0 1px -moz-mac-focusring inset,
               0 0 4px 1px -moz-mac-focusring,

--- a/toolkit/themes/osx/mozapps/extensions/extensions.css
+++ b/toolkit/themes/osx/mozapps/extensions/extensions.css
@@ -792,25 +792,25 @@
 #detail-screenshot {
   -moz-margin-end: 2em;
   max-width: 300px;
   max-height: 300px;
 }
 
 #detail-screenshot[loading] {
   background-image: url("chrome://global/skin/icons/loading_16.png"),
-                    linear-gradient(rgba(255, 255, 255, 0.5), rgba(255, 255, 255, 0));
+                    linear-gradient(rgba(255, 255, 255, 0.5), transparent);
   background-position: 50% 50%;
   background-repeat: no-repeat;
   border-radius: 3px;
 }
 
 #detail-screenshot[loading="error"] {
   background-image: url("chrome://global/skin/media/error.png"),
-                    linear-gradient(rgba(255, 255, 255, 0.5), rgba(255, 255, 255, 0));
+                    linear-gradient(rgba(255, 255, 255, 0.5), transparent);
 }
 
 #detail-desc-container {
   margin-bottom: 2em;
 }
 
 #detail-desc, #detail-fulldesc {
   -moz-margin-start: 6px;
@@ -1147,33 +1147,33 @@ button.button-link:not([disabled="true"]
 .header-button {
   -moz-appearance: none;
   padding: 0 4px;
   margin: 0;
   height: 22px;
   border: 1px solid rgba(60,73,97,0.5);
   border-radius: @toolbarbuttonCornerRadius@;
   box-shadow: inset 0 1px rgba(255,255,255,0.25), 0 1px rgba(255,255,255,0.25);
-  background: linear-gradient(rgba(255,255,255,0.45), rgba(255,255,255,0));
+  background: linear-gradient(rgba(255,255,255,0.45), transparent);
   background-clip: padding-box;
 }
 
 .header-button .toolbarbutton-text {
   display: none;
 }
 
 .header-button[disabled="true"] .toolbarbutton-icon {
   opacity: 0.4;
 }
 
 .header-button:not([disabled="true"]):active:hover,
 .header-button[open="true"] {
   border-color: rgba(45,54,71,0.7);
   box-shadow: inset 0 0 4px rgb(45,54,71), 0 1px rgba(255,255,255,0.25);
-  background-image: linear-gradient(rgba(45,54,71,0.6), rgba(45,54,71,0));
+  background-image: linear-gradient(rgba(45,54,71,0.6), transparent);
 }
 
 /*** telemetry experiments ***/
 
 #detail-experiment-container {
   font-size: 80%;
   margin-bottom: 1em;
 }

--- a/toolkit/themes/windows/global/autocomplete.css
+++ b/toolkit/themes/windows/global/autocomplete.css
@@ -111,20 +111,20 @@ treechildren.autocomplete-treebody::-moz
   -moz-appearance: menuitem is almost right, but the hover effect is not
   transparent and is lighter than desired.
   */
   .autocomplete-richlistitem[selected="true"] {
     color: inherit;
     background-color: transparent;
     /* four gradients for the bevel highlights on each edge, one for blue background */
     background-image:
-      linear-gradient(to bottom, rgba(255,255,255,0.9) 3px, rgba(255,255,255,0) 3px),
-      linear-gradient(to right, rgba(255,255,255,0.5) 3px, rgba(255,255,255,0) 3px),
-      linear-gradient(to left, rgba(255,255,255,0.5) 3px, rgba(255,255,255,0) 3px),
-      linear-gradient(to top, rgba(255,255,255,0.4) 3px, rgba(255,255,255,0) 3px),
+      linear-gradient(to bottom, rgba(255,255,255,0.9) 3px, transparent 3px),
+      linear-gradient(to right, rgba(255,255,255,0.5) 3px, transparent 3px),
+      linear-gradient(to left, rgba(255,255,255,0.5) 3px, transparent 3px),
+      linear-gradient(to top, rgba(255,255,255,0.4) 3px, transparent 3px),
       linear-gradient(to bottom, rgba(163,196,247,0.3), rgba(122,180,246,0.3));
     background-clip: content-box;
     border-radius: 6px;
     outline: 1px solid rgb(124,163,206);
     -moz-outline-radius: 3px;
     outline-offset: -2px;
   }
 }

--- a/toolkit/themes/windows/global/inContentUI.css
+++ b/toolkit/themes/windows/global/inContentUI.css
@@ -33,32 +33,32 @@ html|html {
   }
 }
 
 @media (-moz-windows-glass) {
   *|*:root {
     /* Blame shorlander for this monstrosity. */
     background-image: /* Side gradients */
                       linear-gradient(to right,
-                                      rgba(255,255,255,0.2), rgba(255,255,255,0) 40%,
-                                      rgba(255,255,255,0) 60%, rgba(255,255,255,0.2)),
+                                      rgba(255,255,255,0.2), transparent 40%,
+                                      transparent 60%, rgba(255,255,255,0.2)),
                       /* Aero-style light beams */
                       -moz-linear-gradient(left 32deg,
                                            /* First light beam */
-                                           rgba(255,255,255,0) 19.5%, rgba(255,255,255,0.1) 20%,
+                                           transparent 19.5%, rgba(255,255,255,0.1) 20%,
                                            rgba(255,255,255,0.1) 21.5%, rgba(255,255,255,0.2) 22%,
                                            rgba(255,255,255,0.2) 25.5%, rgba(255,255,255,0.1) 26%,
-                                           rgba(255,255,255,0.1) 27.5%, rgba(255,255,255,0) 28%,
+                                           rgba(255,255,255,0.1) 27.5%, transparent 28%,
                                            /* Second light beam */
-                                           rgba(255,255,255,0) 49.5%, rgba(255,255,255,0.1) 50%,
+                                           transparent 49.5%, rgba(255,255,255,0.1) 50%,
                                            rgba(255,255,255,0.1) 52.5%, rgba(255,255,255,0.2) 53%,
                                            rgba(255,255,255,0.2) 54.5%, rgba(255,255,255,0.1) 55%,
-                                           rgba(255,255,255,0.1) 57.5%, rgba(255,255,255,0) 58%,
+                                           rgba(255,255,255,0.1) 57.5%, transparent 58%,
                                            /* Third light beam */
-                                           rgba(255,255,255,0) 87%, rgba(255,255,255,0.2) 90%),
+                                           transparent 87%, rgba(255,255,255,0.2) 90%),
                       /* Texture */
                       url("chrome://global/skin/inContentUI/background-texture.png");
   }
 }
 %endif
 
 /* Content */
 *|*.main-content {
@@ -83,17 +83,17 @@ html|html {
 @media (-moz-windows-glass) {
   /* Buttons */
   *|button,
   menulist,
   colorpicker[type="button"] {
     -moz-appearance: none;
     color: black;
     padding: 0 5px;
-    background: linear-gradient(rgba(251, 252, 253, 0.95), rgba(246, 247, 248, 0) 49%, 
+    background: linear-gradient(rgba(251, 252, 253, 0.95), transparent 49%, 
                                 rgba(211, 212, 213, 0.45) 51%, rgba(225, 226, 229, 0.3));
     background-clip: padding-box;
     border-radius: 3px;
     border: 1px solid rgba(31, 64, 100, 0.4);
     border-top-color: rgba(31, 64, 100, 0.3);
     box-shadow: 0 0 0 1px rgba(255, 255, 255, 0.25) inset,
                 0 0 2px 1px rgba(255, 255, 255, 0.25) inset;
   }

--- a/toolkit/themes/windows/mozapps/extensions/extensions.css
+++ b/toolkit/themes/windows/mozapps/extensions/extensions.css
@@ -785,25 +785,25 @@
 #detail-screenshot {
   -moz-margin-end: 2em;
   max-width: 300px;
   max-height: 300px;
 }
 
 #detail-screenshot[loading] {
   background-image: url("chrome://global/skin/icons/loading_16.png"),
-                    linear-gradient(rgba(255, 255, 255, 0.5), rgba(255, 255, 255, 0));
+                    linear-gradient(rgba(255, 255, 255, 0.5), transparent);
   background-position: 50% 50%;
   background-repeat: no-repeat;
   border-radius: 3px;
 }
 
 #detail-screenshot[loading="error"] {
   background-image: url("chrome://global/skin/media/error.png"),
-                    linear-gradient(rgba(255, 255, 255, 0.5), rgba(255, 255, 255, 0));
+                    linear-gradient(rgba(255, 255, 255, 0.5), transparent);
 }
 
 #detail-desc-container {
   margin-bottom: 2em;
 }
 
 #detail-desc, #detail-fulldesc {
   -moz-margin-start: 6px;
@@ -1156,17 +1156,17 @@ button.button-link:not([disabled="true"]
   box-shadow: none;
 }
 
 .header-button {
   -moz-appearance: none;
   padding: 1px 3px;
   color: #444;
   text-shadow: 0 0 3px white;
-  background: linear-gradient(rgba(251, 252, 253, 0.95), rgba(246, 247, 248, 0) 49%, 
+  background: linear-gradient(rgba(251, 252, 253, 0.95), transparent 49%, 
                               rgba(211, 212, 213, 0.45) 51%, rgba(225, 226, 229, 0.3));
   background-clip: padding-box;
   border: 1px solid rgba(31, 64, 100, 0.4);
   border-top-color: rgba(31, 64, 100, 0.3);
   box-shadow: 0 0 0 1px rgba(255, 255, 255, 0.25) inset,
               0 0 2px 1px rgba(255, 255, 255, 0.25) inset;
 }