--- a/browser/components/preferences/in-content/subdialogs.js
+++ b/browser/components/preferences/in-content/subdialogs.js
@@ -285,18 +285,18 @@ var gSubDialog = {
     // or their presence will restrict the contents of the <browser>
     // from resizing to a smaller size.
     frame.style.removeProperty("width");
     frame.style.removeProperty("height");
 
     let docEl = frame.contentDocument.documentElement;
     let persistedAttributes = docEl.getAttribute("persist");
     if (!persistedAttributes ||
-        (!persistedAttributes.contains("width") &&
-         !persistedAttributes.contains("height"))) {
+        (!persistedAttributes.includes("width") &&
+         !persistedAttributes.includes("height"))) {
       return;
     }
 
     for (let mutation of mutations) {
       if (mutation.attributeName == "width") {
         docEl.setAttribute("width", docEl.scrollWidth);
       } else if (mutation.attributeName == "height") {
         docEl.setAttribute("height", docEl.scrollHeight);