diff --git a/dom/media/webaudio/test/webaudio.js b/dom/media/webaudio/test/webaudio.js
index f60e8e740420..2ee382ee4eb4 100644
--- a/dom/media/webaudio/test/webaudio.js
+++ b/dom/media/webaudio/test/webaudio.js
@@ -6,7 +6,7 @@ function expectException(func, exceptionCode) {
     func();
   } catch (ex) {
     threw = true;
-    ok(ex instanceof DOMException, "Expect a DOM exception");
+    is(ex.constructor.name, "DOMException", "Expect a DOM exception");
     is(ex.code, exceptionCode, "Expect the correct exception code");
   }
   ok(threw, "The exception was thrown");
diff --git a/editor/libeditor/tests/test_dom_input_event_on_htmleditor.html b/editor/libeditor/tests/test_dom_input_event_on_htmleditor.html
index 05c3edcd1afd..38f9b654795b 100644
--- a/editor/libeditor/tests/test_dom_input_event_on_htmleditor.html
+++ b/editor/libeditor/tests/test_dom_input_event_on_htmleditor.html
@@ -129,7 +129,7 @@ async function runTests() {
           `${aDescription}"beforeinput" event at ${aTestData.action} must be trusted`);
         is(aEvent.target, eventTarget,
           `${aDescription}"beforeinput" event at ${aTestData.action} is fired on unexpected element: ${aEvent.target.tagName}`);
-        ok(aEvent instanceof InputEvent,
+        is(aEvent.constructor.name, "InputEvent",
           `${aDescription}"beforeinput" event at ${aTestData.action} should be dispatched with InputEvent interface`);
         ok(aEvent.bubbles,
           `${aDescription}"beforeinput" event at ${aTestData.action} must be bubbles`);
@@ -148,7 +148,7 @@ async function runTests() {
           `${aDescription}"input" event at ${aTestData.action} must be trusted`);
         is(aEvent.target, eventTarget,
           `${aDescription}"input" event at ${aTestData.action} is fired on unexpected element: ${aEvent.target.tagName}`);
-        ok(aEvent instanceof InputEvent,
+        is(aEvent.constructor.name, "InputEvent",
           `${aDescription}"input" event at ${aTestData.action} should be dispatched with InputEvent interface`);
         ok(!aEvent.cancelable,
           `${aDescription}"input" event at ${aTestData.action} must not be cancelable`);
diff --git a/gfx/layers/apz/test/mochitest/apz_test_native_event_utils.js b/gfx/layers/apz/test/mochitest/apz_test_native_event_utils.js
index a9e17af3c4ed..02f9553cd9ae 100644
--- a/gfx/layers/apz/test/mochitest/apz_test_native_event_utils.js
+++ b/gfx/layers/apz/test/mochitest/apz_test_native_event_utils.js
@@ -202,7 +202,11 @@ function getTargetRect(aTarget) {
   // If the target is the root content window, its origin relative
   // to the visual viewport is (0, 0).
   if (aTarget instanceof Window) {
-    // FIXME: Assert that it's not an iframe window.
+    return rect;
+  }
+  if (aTarget.Window && aTarget instanceof aTarget.Window) {
+    // iframe window
+    // FIXME: Compute proper rect against the root content window
     return rect;
   }
 
diff --git a/js/xpconnect/tests/mochitest/test_bug870423.html b/js/xpconnect/tests/mochitest/test_bug870423.html
index 0d2c6b5ca06c..753f20f6a72a 100644
--- a/js/xpconnect/tests/mochitest/test_bug870423.html
+++ b/js/xpconnect/tests/mochitest/test_bug870423.html
@@ -13,7 +13,9 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=870423
   /** Test for cross-scope instanceof. **/
   SimpleTest.waitForExplicitFinish();
 
-  function go() {
+  async function go() {
+    await SpecialPowers.pushPrefEnv({ set: [["dom.webidl.crosscontext_hasinstance.enabled", false]] });
+
     var sowin = $('soifr').contentWindow;
     var xowin = $('xoifr').contentWindow;
     var xosswin = $('xossifr').contentWindow;
@@ -23,21 +25,21 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=870423
     check(window, sowin, 'Window', function(win) { return win; });
     check(window, sowin, 'Location', function(win) { return win.location; });
 
-    ok(xowin instanceof Window, "Cross-origin instanceof should work");
-    ok(xowin.location instanceof Location, "Cross-origin instanceof should work");
+    ok(!(xowin instanceof Window), "Cross-origin instanceof should fail");
+    ok(!(xowin.location instanceof Location), "Cross-origin instanceof should fail");
 
     // cross-origin same-site.
-    ok(xosswin instanceof Window, "Cross-origin instanceof should work");
-    ok(xosswin.location instanceof Location, "Cross-origin instanceof should work");
+    ok(!(xosswin instanceof Window), "Cross-origin instanceof should fail");
+    ok(!(xosswin.location instanceof Location), "Cross-origin instanceof should fail");
 
     SimpleTest.finish();
   }
 
   function check(win1, win2, constructorName, getInstance) {
-    ok(getInstance(win1) instanceof win2[constructorName],
-       "Cross-Scope instanceof works: " + constructorName + ", " + win1.location + ", " + win2.location);
-    ok(getInstance(win2) instanceof win1[constructorName],
-       "Cross-Scope instanceof works: " + constructorName + ", " + win2.location + ", " + win1.location);
+    ok(!(getInstance(win1) instanceof win2[constructorName]),
+       "Cross-Scope instanceof fails: " + constructorName + ", " + win1.location + ", " + win2.location);
+    ok(!(getInstance(win2) instanceof win1[constructorName]),
+       "Cross-Scope instanceof fails: " + constructorName + ", " + win2.location + ", " + win1.location);
   }
 
   </script>
diff --git a/js/xpconnect/tests/mochitest/test_bug916945.html b/js/xpconnect/tests/mochitest/test_bug916945.html
index 4e481563fcb3..01c342eea16f 100644
--- a/js/xpconnect/tests/mochitest/test_bug916945.html
+++ b/js/xpconnect/tests/mochitest/test_bug916945.html
@@ -22,9 +22,9 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=916945
     // Both same-origin and cross-origin names should be visible if they're set
     // on the iframe element.
     ok('winA' in window, "same-origin named access works");
-    ok(winA instanceof Window, "same-origin named access works");
+    ok(winA instanceof winA.Window, "same-origin named access works");
     ok('winB' in window, "cross-origin named access works when iframe name matches");
-    ok(winB instanceof Window, "cross-origin named access works when iframe name matches");
+    is(winB.parent, window, "cross-origin named access works when iframe name matches");
 
     // Setting the 'name' attribute should propagate to the docshell.
     var ifrB = document.getElementById('ifrB');
@@ -39,7 +39,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=916945
                    'attribute sets propagate to the docshell');
     });
     ok('foo' in window, "names are dynamic if updated via setAttribute");
-    ok(foo instanceof Window, "names are dynamic if updated via setAttribute");
+    is(foo.parent, window, "names are dynamic if updated via setAttribute");
 
     // Setting window.name on the subframe should not propagate to the attribute.
     await SpecialPowers.spawn(ifrB, [], () => {
diff --git a/parser/htmlparser/tests/mochitest/parser_datreader.js b/parser/htmlparser/tests/mochitest/parser_datreader.js
index de310371500f..3ce4557c583d 100644
--- a/parser/htmlparser/tests/mochitest/parser_datreader.js
+++ b/parser/htmlparser/tests/mochitest/parser_datreader.js
@@ -202,7 +202,7 @@ function addLevels(walker, buf, indent) {
       // In the case of template elements, children do not get inserted as
       // children of the template element, instead they are inserted
       // as children of the template content (which is a document fragment).
-      if (walker.currentNode instanceof HTMLTemplateElement) {
+      if (walker.currentNode.constructor.name === "HTMLTemplateElement") {
         buf += indent + "  content\n";
         // Walk through the template content.
         var templateWalker = createFragmentWalker(walker.currentNode.content);