diff --git a/browser/base/content/browser.xhtml b/browser/base/content/browser.xhtml
index 353b42eda20a..52072cbaeb6d 100644
--- a/browser/base/content/browser.xhtml
+++ b/browser/base/content/browser.xhtml
@@ -138,6 +138,10 @@
 
 #include fullscreen-and-pointerlock.inc.xhtml
 
+  <vbox id="browser-bottombox" layer="true">
+    <!-- gNotificationBox will be added here lazily. -->
+  </vbox>
+
   <html:div id="a11y-announcement" role="alert"/>
 
   <!-- Put it at the very end to make sure it's not covered by anything. -->
diff --git a/browser/themes/linux/browser.css b/browser/themes/linux/browser.css
index 5f789ef71589..3f1148ca9536 100644
--- a/browser/themes/linux/browser.css
+++ b/browser/themes/linux/browser.css
@@ -64,6 +64,11 @@
   }
 }
 
+#browser-bottombox {
+  /* opaque for layers optimization */
+  background-color: -moz-Dialog;
+}
+
 /* Bookmark menus */
 .bookmark-item {
   &:is(menu, menuitem) {
diff --git a/browser/themes/windows/browser.css b/browser/themes/windows/browser.css
index 007aec91e089..6aa0a989739f 100644
--- a/browser/themes/windows/browser.css
+++ b/browser/themes/windows/browser.css
@@ -64,6 +64,10 @@
   }
 }
 
+#browser-bottombox:not(:-moz-lwtheme) {
+  background-color: -moz-dialog;
+}
+
 /* Titlebar */
 
 .titlebar-buttonbox {