diff --git a/devtools/client/framework/toolbox-window.xhtml b/devtools/client/framework/toolbox-window.xhtml
index f4daa3bf9057..6021a26e835b 100644
--- a/devtools/client/framework/toolbox-window.xhtml
+++ b/devtools/client/framework/toolbox-window.xhtml
@@ -8,6 +8,7 @@
 <!-- minwidth=50 is sum width of chevron and meatball menu button. -->
 <window
   xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+  xmlns:html="http://www.w3.org/1999/xhtml"
   id="devtools-toolbox-window"
   macanimationtype="document"
   windowtype="devtools:toolbox"
diff --git a/remote/marionette/chrome/test.xhtml b/remote/marionette/chrome/test.xhtml
index c6220a630076..c335a61e2f64 100644
--- a/remote/marionette/chrome/test.xhtml
+++ b/remote/marionette/chrome/test.xhtml
@@ -11,11 +11,9 @@
   title="Title Test"
   windowtype="Test Type"
   xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+  xmlns:html="http://www.w3.org/1999/xhtml"
 >
-  <dialog
-    id="dia"
-    xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
-  >
+  <dialog id="dia">
     <vbox id="things">
       <input
         xmlns="http://www.w3.org/1999/xhtml"
diff --git a/remote/marionette/chrome/test_dialog.xhtml b/remote/marionette/chrome/test_dialog.xhtml
index 0be20b514dda..8b6b4d907dab 100644
--- a/remote/marionette/chrome/test_dialog.xhtml
+++ b/remote/marionette/chrome/test_dialog.xhtml
@@ -10,6 +10,7 @@
 
 <window
   xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+  xmlns:html="http://www.w3.org/1999/xhtml"
   title="&testDialog.title;"
 >
   <dialog id="testDialog" buttons="accept,cancel">
diff --git a/remote/marionette/chrome/test_menupopup.xhtml b/remote/marionette/chrome/test_menupopup.xhtml
index 23ddde041fa5..31a2d2ce1c45 100644
--- a/remote/marionette/chrome/test_menupopup.xhtml
+++ b/remote/marionette/chrome/test_menupopup.xhtml
@@ -9,6 +9,7 @@
 <window
   id="test-window"
   xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+  xmlns:html="http://www.w3.org/1999/xhtml"
 >
   <popupset id="options-popupset">
     <menupopup id="options-menupopup" position="before_end">
diff --git a/toolkit/components/prompts/content/selectDialog.xhtml b/toolkit/components/prompts/content/selectDialog.xhtml
index 27963ebd577d..55fc9e05e79d 100644
--- a/toolkit/components/prompts/content/selectDialog.xhtml
+++ b/toolkit/components/prompts/content/selectDialog.xhtml
@@ -7,7 +7,10 @@
 <?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
 <!DOCTYPE window>
 
-<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+<window
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+  xmlns:html="http://www.w3.org/1999/xhtml"
+>
   <dialog>
     <script src="chrome://global/content/selectDialog.js" />
     <keyset id="dialogKeys" />