diff --git a/dom/bindings/test/test_interfaceToString.html b/dom/bindings/test/test_interfaceToString.html
index c97b2f63b5b7..5c14fced828c 100644
--- a/dom/bindings/test/test_interfaceToString.html
+++ b/dom/bindings/test/test_interfaceToString.html
@@ -20,7 +20,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=742156
 
 /** Test for Bug 742156 **/
 
-var nativeToString = ("" + String.replace).replace("replace", "EventTarget");
+var nativeToString = ("" + String).replace("String", "EventTarget");
 try {
     var eventTargetToString = "" + EventTarget;
     is(eventTargetToString, nativeToString,
diff --git a/dom/encoding/test/test_TextEncoder.js b/dom/encoding/test/test_TextEncoder.js
index 7aa521398dad..531937effb2f 100644
--- a/dom/encoding/test/test_TextEncoder.js
+++ b/dom/encoding/test/test_TextEncoder.js
@@ -182,7 +182,7 @@ function testStreamingOptions()
 }
 
 function arrayFromString(s) {
-  return s.split('').map(function(c){return String.charCodeAt(c)});
+  return s.split('').map(function(c){return c.charCodeAt(0)});
 }
 
 function testArrayOfStrings(test)
diff --git a/dom/security/test/cors/test_CrossSiteXHR.html b/dom/security/test/cors/test_CrossSiteXHR.html
index b3cda3b871f3..714fb2868fb4 100644
--- a/dom/security/test/cors/test_CrossSiteXHR.html
+++ b/dom/security/test/cors/test_CrossSiteXHR.html
@@ -673,7 +673,7 @@ function runTest() {
       reqHeaders =
         escape(Object.keys(test.headers)
                .filter(isUnsafeHeader)
-               .map(String.toLowerCase)
+               .map(s => s.toLowerCase())
                .sort()
                .join(","));
       req.url += reqHeaders ? "&requestHeaders=" + reqHeaders : "";
diff --git a/dom/tests/mochitest/fetch/test_fetch_cors.js b/dom/tests/mochitest/fetch/test_fetch_cors.js
index ac83d050d243..412bb1288e18 100644
--- a/dom/tests/mochitest/fetch/test_fetch_cors.js
+++ b/dom/tests/mochitest/fetch/test_fetch_cors.js
@@ -759,7 +759,7 @@ function testModeCors() {
       reqHeaders =
         escape(Object.keys(test.headers)
                .filter(isUnsafeHeader)
-               .map(String.toLowerCase)
+               .map(s => s.toLowerCase())
                .sort()
                .join(","));
       req.url += reqHeaders ? "&requestHeaders=" + reqHeaders : "";