--- a/browser/components/extensions/ext-theme.js
+++ b/browser/components/extensions/ext-theme.js
@@ -203,16 +203,17 @@ const kBrowserOverrideIconsVarMap = new 
   ["bookmark_star", [
     ["#bookmarks-menu-button", {"list-style-image": "--bookmark_star-icon", "-moz-image-region": "rect(0, 18px, 18px, 0)"}],
   ]],
   ["bookmark_menu", [
     ["#bookmarks-menu-button[cui-areatype='toolbar'] > .toolbarbutton-menubutton-dropmarker > .dropmarker-icon", {"list-style-image": "--bookmark_menu-icon", "-moz-image-region": "rect(0, 18px, 18px, 0)"}],
   ]],
   ["downloads", [
     ["#downloads-button", {"list-style-image": "--downloads-icon", "-moz-image-region": "rect(0, 18px, 18px, 0)", "background-image": "none"}],
+    ["#downloads-indicator-icon", {"background": "var(--downloads-icon) center no-repeat"}],
   ]],
   ["home", [
     ["#home-button", {"list-style-image": "--home-icon", "-moz-image-region": "rect(0, 18px, 18px, 0)"}],
   ]],
   ["app_menu", [
     ["#PanelUI-menu-button", {"list-style-image": "--app_menu-icon", "-moz-image-region": "rect(0, 18px, 18px, 0)"}],
   ]],
   ["cut", [

--- a/browser/components/extensions/test/browser/browser_ext_theme_icons.js
+++ b/browser/components/extensions/test/browser/browser_ext_theme_icons.js
@@ -130,16 +130,25 @@ add_task(function* testCustomizedIcons()
 
     yield extension.startup();
 
     for (let button of buttons) {
       verifyButtonWithCustomStyling(button[1], button[0],
         `The ${button[1]} should have it's icon customized in the toolbar`);
     }
 
+    EventUtils.synthesizeMouseAtCenter(document.getElementById("downloads-button"), {});
+    yield BrowserTestUtils.waitForCondition(() => document.getElementById("downloads-indicator-icon"),
+      "downloads-indicator-icon is added to the DOM");
+    let downloadsIndicatorIcon = document.getElementById("downloads-indicator-icon");
+    let backgroundImage = getComputedStyle(downloadsIndicatorIcon).backgroundImage;
+    info(`backgroundImage for #downloads-indicator-icon is ${backgroundImage}`);
+    ok(backgroundImage.includes(iconForButton("downloads")),
+      "The #downloads-indicator-icon should have it's icon customized in the toolbar");
+
     for (let button of buttons) {
       if (button[2]) {
         CustomizableUI.addWidgetToArea(button[2], CustomizableUI.AREA_PANEL);
       }
     }
 
     yield PanelUI.show();