Bug 1501478 - Fix payment dialog in browser.xhtml. r=MattN

Differential Revision: https://phabricator.services.mozilla.com/D9602

--HG--
extra : moz-landing-system : lando

browser/components/payments/paymentUIService.js

@@ -79,7 +79,7 @@ PaymentUIService.prototype = {
       merchantBrowser.setAttribute("tabmodalPromptShowing", "true");
 
       // Darken the merchant content area.
-      let tabModalBackground = chromeWindow.document.createElement("box");
+      let tabModalBackground = chromeWindow.document.createXULElement("box");
       tabModalBackground.classList.add("tabModalBackground", "paymentDialogBackground");
       // Insert the same way as <tabmodalprompt>.
       merchantBrowser.parentNode.insertBefore(tabModalBackground,