diff --git a/toolkit/components/processsingleton/MainProcessSingleton.js b/toolkit/components/processsingleton/MainProcessSingleton.js
index d04f2e39cbd2..e6a034eeac41 100644
--- a/toolkit/components/processsingleton/MainProcessSingleton.js
+++ b/toolkit/components/processsingleton/MainProcessSingleton.js
@@ -73,13 +73,15 @@ MainProcessSingleton.prototype = {
     }
 
     case "document-element-inserted":
-      // Set up Custom Elements for XUL windows before anything else happens
-      // in the document. Anything loaded here should be considered part of
-      // core XUL functionality. Any window-specific elements can be registered
-      // via <script> tags at the top of individual documents.
+      // Set up Custom Elements for XUL and XHTML documents before anything else
+      // happens. Anything loaded here should be considered part of core XUL functionality.
+      // Any window-specific elements can be registered via <script> tags at the
+      // top of individual documents.
       const doc = subject;
-      if (doc.nodePrincipal.isSystemPrincipal &&
-          doc.contentType == "application/vnd.mozilla.xul+xml") {
+      if (doc.nodePrincipal.isSystemPrincipal && (
+            doc.contentType == "application/vnd.mozilla.xul+xml" ||
+            doc.contentType == "application/xhtml+xml"
+        )) {
         Services.scriptloader.loadSubScript(
           "chrome://global/content/customElements.js", doc.ownerGlobal);
       }
diff --git a/toolkit/content/tests/chrome/chrome.ini b/toolkit/content/tests/chrome/chrome.ini
index 91cb17eed6f9..a937ab8b2da0 100644
--- a/toolkit/content/tests/chrome/chrome.ini
+++ b/toolkit/content/tests/chrome/chrome.ini
@@ -14,6 +14,7 @@ support-files =
   bug624329_window.xul
   dialog_dialogfocus.xul
   dialog_dialogfocus2.xul
+  file_empty.xhtml
   file_about_networking_wsh.py
   file_autocomplete_with_composition.js
   file_editor_with_autocomplete.js
diff --git a/toolkit/content/tests/chrome/file_empty.xhtml b/toolkit/content/tests/chrome/file_empty.xhtml
new file mode 100644
index 000000000000..cda0d080bd4a
--- /dev/null
+++ b/toolkit/content/tests/chrome/file_empty.xhtml
@@ -0,0 +1,3 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<html xmlns='http://www.w3.org/1999/xhtml'>
+</html>
\ No newline at end of file
diff --git a/toolkit/content/tests/chrome/test_custom_element_base.xul b/toolkit/content/tests/chrome/test_custom_element_base.xul
index 024f533202bc..14e7f3509636 100644
--- a/toolkit/content/tests/chrome/test_custom_element_base.xul
+++ b/toolkit/content/tests/chrome/test_custom_element_base.xul
@@ -19,10 +19,19 @@
 
   SimpleTest.waitForExplicitFinish();
 
-  function runTests() {
+  async function runTests() {
     ok(MozXULElement, "MozXULElement defined on the window");
     testParseXULToFragment();
     testCustomInterface();
+
+    let htmlWin = await new Promise(resolve => {
+      let htmlIframe = document.createElement("iframe");
+      htmlIframe.src = "file_empty.xhtml";
+      htmlIframe.onload = () => resolve(htmlIframe.contentWindow);
+      document.documentElement.appendChild(htmlIframe);
+    });
+
+    ok(htmlWin.MozXULElement, "MozXULElement defined on a chrome HTML window");
     SimpleTest.finish();
   }
 
