diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index 950820b82f70..fe6c250b2d69 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -10119,12 +10119,8 @@ nsGlobalWindow::ShowModalDialog(JSContext* aCx, const nsAString& aUrl,
 
   JS::Rooted<JS::Value> result(aCx);
   if (retVal) {
-    JS::Rooted<JSObject*> global(aCx, JS::CurrentGlobalOrNull(aCx));
-    if (!global) {
-      global = FastGetGlobalJSObject();
-    }
     aError = nsContentUtils::XPConnect()->VariantToJS(aCx,
-                                                      global,
+                                                      FastGetGlobalJSObject(),
                                                       retVal, aRetval);
   } else {
     aRetval.setNull();
diff --git a/dom/base/test/test_dialogArguments.html b/dom/base/test/test_dialogArguments.html
index 682f61e0b294..70a091d0005e 100644
--- a/dom/base/test/test_dialogArguments.html
+++ b/dom/base/test/test_dialogArguments.html
@@ -16,16 +16,8 @@
 */
 SimpleTest.waitForExplicitFinish();
 
-async function openModal() {
-	await SpecialPowers.pushPrefEnv({ set: [[
-		"dom.disable_window_showModalDialog", false ]] });
-
-	let iframe = document.createElement("iframe");
-	document.body.appendChild(iframe);
-	await new Promise(resolve => {
-		iframe.addEventListener("load", resolve);
-	});
-	iframe.contentWindow.showModalDialog("javascript:opener.winRef = window; \
+function openModal() {
+	showModalDialog("javascript:opener.winRef = window; \
 		window.opener.setTimeout(\'winRef.dialogArguments;\', 0);\
 		window.close();");
 
diff --git a/dom/base/test/test_mutationobservers.html b/dom/base/test/test_mutationobservers.html
index 6a5b43861075..f26de3728a63 100644
--- a/dom/base/test/test_mutationobservers.html
+++ b/dom/base/test/test_mutationobservers.html
@@ -487,14 +487,7 @@ function testSyncXHR2() {
   then(testModalDialog);
 }
 
-async function testModalDialog() {
-  await SpecialPowers.pushPrefEnv({ set: [[ "dom.disable_window_showModalDialog", false ]] });
-  let iframe = document.createElement("iframe");
-  document.body.appendChild(iframe);
-  await new Promise(resolve => {
-    iframe.addEventListener("load", resolve);
-  });
-  window.showModalDialog = iframe.contentWindow.showModalDialog;
+function testModalDialog() {
   var didHandleCallback = false;
   div.innerHTML = "<span>1</span><span>2</span>";
   m = new M(function(records, observer) {
diff --git a/dom/html/test/test_bug391777.html b/dom/html/test/test_bug391777.html
index 10f230684736..aa01a45de73f 100644
--- a/dom/html/test/test_bug391777.html
+++ b/dom/html/test/test_bug391777.html
@@ -14,23 +14,11 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=391777
 <script class="testbody" type="text/javascript">
 
 /** Test for Bug 391777 **/
-SimpleTest.waitForExplicitFinish();
-
-SpecialPowers.pushPrefEnv({ set: [[ "dom.disable_window_showModalDialog", false ]] }, doTest);
-async function doTest() {
-  let iframe = document.createElement("iframe");
-  document.body.appendChild(iframe);
-  await new Promise(resolve => {
-    iframe.addEventListener("load", resolve);
-  });
-  window.showModalDialog = iframe.contentWindow.showModalDialog;
-  var arg = {};
-  arg.testVal = "foo";
-  var result = window.showModalDialog("javascript:window.returnValue = window.dialogArguments.testVal; window.close(); 'This window should close on its own.';", arg);
-  ok(true, "We should get here without user interaction");
-  is(result, "foo", "Unexpected result from showModalDialog");
-  SimpleTest.finish();
-}
+var arg = {};
+arg.testVal = "foo";
+var result = window.showModalDialog("javascript:window.returnValue = window.dialogArguments.testVal; window.close(); 'This window should close on its own.';", arg);
+ok(true, "We should get here without user interaction");
+is(result, "foo", "Unexpected result from showModalDialog");
 
 </script>
 </body>
diff --git a/dom/html/test/test_iframe_sandbox_modal.html b/dom/html/test/test_iframe_sandbox_modal.html
index a1bc18191d8e..2e670239feb6 100644
--- a/dom/html/test/test_iframe_sandbox_modal.html
+++ b/dom/html/test/test_iframe_sandbox_modal.html
@@ -109,9 +109,7 @@ function doTest() {
   // file_iframe_j_if3.html has an ok() function that calls window.parent.ok_wrapper.
 }
 
-let loaded = new Promise(resolve => { addLoadEvent(resolve) });
-let prefSet = SpecialPowers.pushPrefEnv({ set: [[ "dom.disable_window_showModalDialog", false ]] });
-Promise.all([ loaded, prefSet ]).then(doTest);
+addLoadEvent(doTest);
 </script>
 
 <body>
diff --git a/dom/tests/mochitest/bugs/mochitest.ini b/dom/tests/mochitest/bugs/mochitest.ini
index d8256968e887..544376d7bec9 100644
--- a/dom/tests/mochitest/bugs/mochitest.ini
+++ b/dom/tests/mochitest/bugs/mochitest.ini
@@ -59,7 +59,7 @@ skip-if = toolkit == 'android'
 [test_bug265203.html]
 [test_bug291377.html]
 [test_bug291653.html]
-skip-if = e10s || toolkit == 'android' #TIMED_OUT
+skip-if = toolkit == 'android' #TIMED_OUT
 [test_bug304459.html]
 [test_bug308856.html]
 [test_bug327891.html]
@@ -82,7 +82,7 @@ skip-if = toolkit == 'android' #TIMED_OUT
 [test_bug400204.html]
 [test_bug404748.html]
 [test_bug406375.html]
-skip-if = e10s || toolkit == 'android'
+skip-if = toolkit == 'android'
 [test_bug414291.html]
 tags = openwindow
 [test_bug427744.html]
@@ -90,20 +90,20 @@ skip-if = toolkit == 'android'
 [test_bug42976.html]
 [test_bug430276.html]
 [test_bug437361.html]
-skip-if = e10s || toolkit == 'android'
+skip-if = toolkit == 'android'
 [test_bug440572.html]
 [test_bug456151.html]
 [test_bug458091.html]
 [test_bug459848.html]
 [test_bug465263.html]
 [test_bug479143.html]
-skip-if = e10s || toolkit == 'android'
+skip-if = toolkit == 'android'
 [test_bug484775.html]
 [test_bug492925.html]
 [test_bug49312.html]
 [test_bug495219.html]
 [test_bug504862.html]
-skip-if = e10s || toolkit == 'android' #RANDOM
+skip-if = toolkit == 'android' #RANDOM
 [test_bug529328.html]
 [test_bug531176.html]
 [test_bug531542.html]
diff --git a/dom/tests/mochitest/bugs/test_bug291653.html b/dom/tests/mochitest/bugs/test_bug291653.html
index 0237c6bff4a8..541ceab93a67 100644
--- a/dom/tests/mochitest/bugs/test_bug291653.html
+++ b/dom/tests/mochitest/bugs/test_bug291653.html
@@ -25,7 +25,13 @@ var secondListenerDidRun = false;
 
 var w;
 function start() {
-  w = window.open("file_bug291653.html", "foo", "width=300,height=300");
+  if ("showModalDialog" in window) {
+    w = window.open("file_bug291653.html", "foo", "width=300,height=300");
+  } else {
+    // window.showModalDialog doesn't exist in e10s mode, nothing to do in this test.
+    ok(true, "nothing to do in e10s mode");
+    SimpleTest.finish();
+  }
 }
 
 function closeTest() {
@@ -42,7 +48,7 @@ function end() {
   setTimeout("closeTest()", 500);
 }
 
-SpecialPowers.pushPrefEnv({ set: [[ "dom.disable_window_showModalDialog", false ]] }, start);
+start();
 
 </script>
 </pre>
diff --git a/dom/tests/mochitest/bugs/test_bug406375.html b/dom/tests/mochitest/bugs/test_bug406375.html
index a2e41f368603..2cd459ffa497 100644
--- a/dom/tests/mochitest/bugs/test_bug406375.html
+++ b/dom/tests/mochitest/bugs/test_bug406375.html
@@ -22,16 +22,10 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=406375
 
 SimpleTest.waitForExplicitFinish();
 
-async function runTest() {
-  await SpecialPowers.pushPrefEnv({ set: [[ "dom.disable_window_showModalDialog", false ]] });
-  let iframe = document.createElement("iframe");
-  document.body.appendChild(iframe);
-  await new Promise(resolve => {
-    iframe.addEventListener("load", resolve);
-  });
-  window.showModalDialog = iframe.contentWindow.showModalDialog;
-
-  window.showModalDialog("file_bug406375.html");
+function runTest() {
+  if ("showModalDialog" in window) {
+    window.showModalDialog("file_bug406375.html");
+  }
   ok(true, "This test should not hang");
 
   SimpleTest.finish();
diff --git a/dom/tests/mochitest/bugs/test_bug414291.html b/dom/tests/mochitest/bugs/test_bug414291.html
index e50038216a95..119c1d086bec 100644
--- a/dom/tests/mochitest/bugs/test_bug414291.html
+++ b/dom/tests/mochitest/bugs/test_bug414291.html
@@ -13,7 +13,6 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=414291
 <script class="testbody" type="text/javascript">
 
 /** Test for Bug 414291 **/
-SimpleTest.waitForExplicitFinish();
 
 var result1 = 0;
 var result2 = 0;
@@ -25,20 +24,9 @@ is(result1, 0, "window should not be opened either as modal or loaded synchronou
 window.open("file2_bug414291.html", "w2", "modal=yes");
 is(result2, 0, "window should not be opened either as modal or data loaded synchronously.");
 
-SpecialPowers.pushPrefEnv({ set: [[ "dom.disable_window_showModalDialog", false ]] }, runModalTest);
-async function runModalTest() {
-  let iframe = document.createElement("iframe");
-  document.body.appendChild(iframe);
-  await new Promise(resolve => {
-    iframe.addEventListener("load", resolve);
-  });
-  window.showModalDialog = iframe.contentWindow.showModalDialog;
-
-  if (window.showModalDialog) {
-    result3 = window.showModalDialog("file3_bug414291.html");
-    is(result3, 3, "window should be opened as modal.");
-  }
-  SimpleTest.finish();
+if (window.showModalDialog) {
+  result3 = window.showModalDialog("file3_bug414291.html");
+  is(result3, 3, "window should be opened as modal.");
 }
 
 </script>
diff --git a/dom/tests/mochitest/bugs/test_bug437361.html b/dom/tests/mochitest/bugs/test_bug437361.html
index c1ed6135562e..1277f0ff4695 100644
--- a/dom/tests/mochitest/bugs/test_bug437361.html
+++ b/dom/tests/mochitest/bugs/test_bug437361.html
@@ -32,41 +32,32 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=437361
   }
 
   function test(disableOpen, exceptionExpected, testFn, errorMsg) {
-    var oldPrefVal = SpecialPowers.getBoolPref("dom.disable_open_during_load");
-    try {
-      SpecialPowers.setBoolPref("dom.disable_open_during_load", disableOpen);
-      testFn();
-      ok(!exceptionExpected, errorMsg);
-    } catch (_) {
-      ok(exceptionExpected, errorMsg);
-    }
-    finally {
-      SpecialPowers.setBoolPref("dom.disable_open_during_load", oldPrefVal);
+    if ("showModalDialog" in window) {
+      var oldPrefVal = SpecialPowers.getBoolPref("dom.disable_open_during_load");
+      try {
+        SpecialPowers.setBoolPref("dom.disable_open_during_load", disableOpen);
+        testFn();
+        ok(!exceptionExpected, errorMsg);
+      } catch (_) {
+        ok(exceptionExpected, errorMsg);
+      }
+      finally {
+        SpecialPowers.setBoolPref("dom.disable_open_during_load", oldPrefVal);
+      }
+    } else {
+      ok(true, "nothing to do in e10s mode");
     }
   }
 
-  SimpleTest.waitForExplicitFinish();
+  test(true, false, testModalDialogBlockedCleanly,
+       "Blocked showModalDialog caused an exception.");
+       
+  test(false, false, testModalDialogAllowed,
+       "showModalDialog was blocked even though dom.disable_open_during_load was false.");
 
-  SpecialPowers.pushPrefEnv({ set: [[ "dom.disable_window_showModalDialog", false ]] }, doTest);
-  async function doTest() {
-    let iframe = document.createElement("iframe");
-    document.body.appendChild(iframe);
-    await new Promise(resolve => {
-      iframe.addEventListener("load", resolve);
-    });
-    window.showModalDialog = iframe.contentWindow.showModalDialog;
+  test(false, true, testOtherExceptionsNotTrapped,
+       "Incorrectly suppressed insecure showModalDialog exception.");
 
-    test(true, false, testModalDialogBlockedCleanly,
-         "Blocked showModalDialog caused an exception.");
-
-    test(false, false, testModalDialogAllowed,
-         "showModalDialog was blocked even though dom.disable_open_during_load was false.");
-
-    test(false, true, testOtherExceptionsNotTrapped,
-         "Incorrectly suppressed insecure showModalDialog exception.");
-
-    SimpleTest.finish();
-  }
   </script>
 </head>
 <body>
diff --git a/dom/tests/mochitest/bugs/test_bug479143.html b/dom/tests/mochitest/bugs/test_bug479143.html
index 6227259ed09a..03db4ddea4a3 100644
--- a/dom/tests/mochitest/bugs/test_bug479143.html
+++ b/dom/tests/mochitest/bugs/test_bug479143.html
@@ -18,32 +18,25 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=479143
 
   SimpleTest.waitForExplicitFinish();
 
-  SpecialPowers.pushPrefEnv({ set: [[ "dom.disable_window_showModalDialog", false ]] }, doTest);
-  async function doTest() {
-    await SpecialPowers.pushPrefEnv({ set: [[ "dom.disable_window_showModalDialog", false ]] });
-    let iframe = document.createElement("iframe");
-    document.body.appendChild(iframe);
-    await new Promise(resolve => {
-      iframe.addEventListener("load", resolve);
-    });
-    window.showModalDialog = iframe.contentWindow.showModalDialog;
+  setTimeout(function() {
+    if ("showModalDialog" in window) {
+      var interval = setInterval(function() { var i = 0; i++; }, 10);
 
-    var interval = setInterval(function() { var i = 0; i++; }, 10);
+      var xhr = new XMLHttpRequest();
+      xhr.open("GET", "test_bug479143.html", false);
+      xhr.send(null);
 
-    var xhr = new XMLHttpRequest();
-    xhr.open("GET", "test_bug479143.html", false);
-    xhr.send(null);
+      window.showModalDialog("javascript:" +
+                             "setTimeout(function() { window.close(); }, 1000);",
+                             null);
 
-    window.showModalDialog("javascript:" +
-                           "setTimeout(function() { window.close(); }, 1000);",
-                           null);
-
-    clearInterval(interval);
+      clearInterval(interval);
+    }
 
     ok(true, "did not crash");
 
     SimpleTest.finish();
-  }
+  }, 0);
 
 </script>
 </pre>
diff --git a/dom/tests/mochitest/bugs/test_bug504862.html b/dom/tests/mochitest/bugs/test_bug504862.html
index 009c80ae3928..947d18171fcf 100644
--- a/dom/tests/mochitest/bugs/test_bug504862.html
+++ b/dom/tests/mochitest/bugs/test_bug504862.html
@@ -19,26 +19,22 @@ function onMsgRcv(event)
   is(event.data, "args: undefined", "Unexpected cross origin dialog arguments.");
 }
 
-async function runTest() {
-  await SpecialPowers.pushPrefEnv({ set: [[ "dom.disable_window_showModalDialog", false ]] });
-  let iframe = document.createElement("iframe");
-  document.body.appendChild(iframe);
-  await new Promise(resolve => {
-    iframe.addEventListener("load", resolve);
-  });
-  window.showModalDialog = iframe.contentWindow.showModalDialog;
+function runTest() {
+  if ("showModalDialog" in window) {
+    window.addEventListener("message", onMsgRcv);
 
-  window.addEventListener("message", onMsgRcv);
+    var result = window.showModalDialog("file_bug504862.html", "my args");
+    // NB: We used to clear returnValue on each navigation, but now we do a
+    // security check on access, so we can safely make returnValue live on
+    // the browsing context, per spec.
+    is(result, 3, "window sees previous dialog documents return value.");
 
-  var result = window.showModalDialog("file_bug504862.html", "my args");
-  // NB: We used to clear returnValue on each navigation, but now we do a
-  // security check on access, so we can safely make returnValue live on
-  // the browsing context, per spec.
-  is(result, 3, "window sees previous dialog documents return value.");
+    result = window.showModalDialog("http://test1.example.com/tests/dom/tests/mochitest/bugs/file_bug504862.html", "my args");
 
-  result = window.showModalDialog("http://test1.example.com/tests/dom/tests/mochitest/bugs/file_bug504862.html", "my args");
-
-  is(result, undefined, "Able to see return value from cross origin dialog.");
+    is(result, undefined, "Able to see return value from cross origin dialog.");
+  } else {
+    ok(true, "nothing to do in e10s mode");
+  }
 
   SimpleTest.finish();
 }
diff --git a/dom/tests/mochitest/bugs/test_bug61098.html b/dom/tests/mochitest/bugs/test_bug61098.html
index f28b5fe2c462..6543f59c36fe 100644
--- a/dom/tests/mochitest/bugs/test_bug61098.html
+++ b/dom/tests/mochitest/bugs/test_bug61098.html
@@ -220,12 +220,11 @@ var expectedState;
 
 function runtests()
 {
-  SpecialPowers.pushPrefEnv({'set': [["dom.successive_dialog_time_limit", 3],
-                                     ["dom.disable_window_showModalDialog", false]]},
+  SpecialPowers.pushPrefEnv({'set': [["dom.successive_dialog_time_limit", 3]]},
                             runtestsInner);
 }
 
-async function runtestsInner()
+function runtestsInner()
 {
   registerMockPromptService();
 
@@ -306,13 +305,6 @@ async function runtestsInner()
 
   w.close();
 
-  let iframe = document.createElement("iframe");
-  document.body.appendChild(iframe);
-  await new Promise(resolve => {
-    iframe.addEventListener("load", resolve);
-  });
-  window.showModalDialog = iframe.contentWindow.showModalDialog;
-
   // Test that showModalDialog() works normally and then gets blocked
   // on the second call.
   if (window.showModalDialog) {
diff --git a/dom/tests/mochitest/general/mochitest.ini b/dom/tests/mochitest/general/mochitest.ini
index a0bec29bc8af..4e5f6bf6c1b8 100644
--- a/dom/tests/mochitest/general/mochitest.ini
+++ b/dom/tests/mochitest/general/mochitest.ini
@@ -124,7 +124,8 @@ support-files = test_offsets.js
 skip-if = toolkit == 'android' # bug 1230232 - Mouse doesn't select in the same way
 [test_showModalDialog.html]
 skip-if = e10s || toolkit == 'android' #Don't run modal tests on Android
-[test_showModalDialog_removed.html]
+[test_showModalDialog_e10s.html]
+run-if = e10s
 [test_storagePermissionsAccept.html]
 [test_storagePermissionsLimitForeign.html]
 [test_storagePermissionsReject.html]
diff --git a/dom/tests/mochitest/general/test_showModalDialog.html b/dom/tests/mochitest/general/test_showModalDialog.html
index 46126af5ab31..511c79e639de 100644
--- a/dom/tests/mochitest/general/test_showModalDialog.html
+++ b/dom/tests/mochitest/general/test_showModalDialog.html
@@ -24,40 +24,27 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=862918
     win.wrappedJSObject.go();
   };
 
-  SimpleTest.waitForExplicitFinish();
+  var someObj = { foo: 42, bar: "hi"};
+  var xurl = location.toString()
+                     .replace('mochi.test:8888', 'example.org')
+                     .replace('test_showModal', 'file_showModal');
+  if (xurl.indexOf('?') != -1)
+    xurl = xurl.substring(0, xurl.indexOf('?'));
+  is(showModalDialog('file_showModalDialog.html'), "rv: undefined");
+  is(showModalDialog(xurl), undefined);
+  is(showModalDialog('file_showModalDialog.html', 3), "rv: 3");
+  is(showModalDialog(xurl, 3), undefined);
+  is(showModalDialog('file_showModalDialog.html', someObj), "rv: " + someObj);
+  is(showModalDialog(xurl, someObj), undefined);
 
-  SpecialPowers.pushPrefEnv({ set: [[ "dom.disable_window_showModalDialog", false ]] }, doTest);
-  async function doTest() {
-    let iframe = document.createElement("iframe");
-    document.body.appendChild(iframe);
-    await new Promise(resolve => {
-      iframe.addEventListener("load", resolve);
-    });
-    window.showModalDialog = iframe.contentWindow.showModalDialog;
+  // Test sequential navigations.
+  is(showModalDialog('file_showModalDialog.html?http://mochi.test:8888', 4),
+                     'rv: 4');
+  is(showModalDialog('file_showModalDialog.html?http://example.com', 4), undefined);
+  is(showModalDialog('file_showModalDialog.html?http://example.com,http://mochi.test:8888', 4), 'rv: 4');
 
-    var someObj = { foo: 42, bar: "hi"};
-    var xurl = location.toString()
-                       .replace('mochi.test:8888', 'example.org')
-                       .replace('test_showModal', 'file_showModal');
-    if (xurl.indexOf('?') != -1)
-      xurl = xurl.substring(0, xurl.indexOf('?'));
-    is(window.showModalDialog('file_showModalDialog.html'), "rv: undefined");
-    is(window.showModalDialog(xurl), undefined);
-    is(window.showModalDialog('file_showModalDialog.html', 3), "rv: 3");
-    is(window.showModalDialog(xurl, 3), undefined);
-    is(window.showModalDialog('file_showModalDialog.html', someObj), "rv: " + someObj);
-    is(window.showModalDialog(xurl, someObj), undefined);
-
-    // Test sequential navigations.
-    is(window.showModalDialog('file_showModalDialog.html?http://mochi.test:8888', 4),
-                       'rv: 4');
-    is(window.showModalDialog('file_showModalDialog.html?http://example.com', 4), undefined);
-    is(window.showModalDialog('file_showModalDialog.html?http://example.com,http://mochi.test:8888', 4), 'rv: 4');
-
-    // This test used to assert after gc. Make sure it doesn't.
-    SpecialPowers.gc();
-    SimpleTest.finish();
-  }
+  // This test used to assert after gc. Make sure it doesn't.
+  SpecialPowers.gc();
 
   </script>
 </head>
diff --git a/dom/tests/mochitest/general/test_showModalDialog_removed.html b/dom/tests/mochitest/general/test_showModalDialog_e10s.html
similarity index 94%
rename from dom/tests/mochitest/general/test_showModalDialog_removed.html
rename to dom/tests/mochitest/general/test_showModalDialog_e10s.html
index 6222f5eadfb4..5a6f3d7f33f6 100644
--- a/dom/tests/mochitest/general/test_showModalDialog_removed.html
+++ b/dom/tests/mochitest/general/test_showModalDialog_e10s.html
@@ -20,7 +20,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1077002
 <pre id="test">
 <script type="application/javascript">
 
-/** Test for showModalDialog unavailability in Firefox **/
+/** Test for showModalDialog unavailability in e10s **/
 
 // NB: This test runs in e10s only. In e10s showModalDialog should not
 // exist.
diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index 07c68e8b900b..599ff1cd5643 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -1221,7 +1221,6 @@ pref("dom.disable_window_open_feature.menubar",     false);
 pref("dom.disable_window_open_feature.resizable",   true);
 pref("dom.disable_window_open_feature.minimizable", false);
 pref("dom.disable_window_open_feature.status",      true);
-pref("dom.disable_window_showModalDialog",          true);
 
 pref("dom.allow_scripts_to_close_windows",          false);
 
diff --git a/testing/web-platform/meta/html/browsers/the-window-object/historical.window.js.ini b/testing/web-platform/meta/html/browsers/the-window-object/historical.window.js.ini
new file mode 100644
index 000000000000..bbfc462890b1
--- /dev/null
+++ b/testing/web-platform/meta/html/browsers/the-window-object/historical.window.js.ini
@@ -0,0 +1,15 @@
+[historical.window.html]
+  type: testharness
+  [showModalDialog() has been removed from the platform]
+    expected:
+      if debug and not e10s and (os == "linux") and (version == "Ubuntu 16.04") and (processor == "x86") and (bits == 32): FAIL
+      if not debug and not e10s and (os == "linux") and (version == "Ubuntu 16.04") and (processor == "x86_64") and (bits == 64): FAIL
+      if debug and not e10s and (os == "win") and (version == "6.1.7601") and (processor == "x86") and (bits == 32): FAIL
+      if not debug and not e10s and (os == "linux") and (version == "Ubuntu 16.04") and (processor == "x86") and (bits == 32): FAIL
+      if not debug and not e10s and (os == "win") and (version == "6.1.7601") and (processor == "x86") and (bits == 32): FAIL
+      if not debug and not e10s and (os == "win") and (version == "6.2.9200") and (processor == "x86_64") and (bits == 64): FAIL
+      if debug and not e10s and (os == "win") and (version == "6.2.9200") and (processor == "x86_64") and (bits == 64): FAIL
+      if debug and not e10s and (os == "linux") and (version == "Ubuntu 16.04") and (processor == "x86_64") and (bits == 64): FAIL
+      if not debug and not e10s and (os == "mac") and (version == "OS X 10.10.5") and (processor == "x86_64") and (bits == 64): FAIL
+      if not debug and not e10s and (os == "win") and (version == "10.0.15063") and (processor == "x86_64") and (bits == 64): FAIL
+
diff --git a/testing/web-platform/meta/html/browsers/the-window-object/security-window/window-security.html.ini b/testing/web-platform/meta/html/browsers/the-window-object/security-window/window-security.html.ini
index a02fb9ee4caf..aca24d5fa60d 100644
--- a/testing/web-platform/meta/html/browsers/the-window-object/security-window/window-security.html.ini
+++ b/testing/web-platform/meta/html/browsers/the-window-object/security-window/window-security.html.ini
@@ -8,3 +8,8 @@
 
   [A SecurityError exception must be thrown when window.onmousewheel is accessed from a different origin.]
     expected: FAIL
+
+  [A SecurityError exception must be thrown when window.showModalDialog is accessed from a different origin.]
+    disabled:
+      if e10s: https://bugzilla.mozilla.org/show_bug.cgi?id=981796
+
diff --git a/testing/web-platform/meta/html/dom/interfaces.html.ini b/testing/web-platform/meta/html/dom/interfaces.html.ini
index c85c4854ebca..79c22ac52868 100644
--- a/testing/web-platform/meta/html/dom/interfaces.html.ini
+++ b/testing/web-platform/meta/html/dom/interfaces.html.ini
@@ -1477,6 +1477,10 @@
   [DataTransfer interface object length]
     expected: FAIL
 
+  [Window interface: operation showModalDialog(DOMString,any)]
+    disabled:
+      if e10s: https://bugzilla.mozilla.org/show_bug.cgi?id=981796
+
   [Window interface: attribute oncancel]
     expected: FAIL
 
@@ -1492,6 +1496,10 @@
   [Window interface: operation createImageBitmap(ImageBitmapSource,long,long,long,long)]
     expected: FAIL
 
+  [Window interface: window must inherit property "showModalDialog" with the proper type (34)]
+    disabled:
+      if e10s: https://bugzilla.mozilla.org/show_bug.cgi?id=981796
+
   [Window interface: window must inherit property "oncancel" with the proper type (42)]
     expected: FAIL
 
@@ -1504,6 +1512,10 @@
   [Window interface: window must inherit property "onsort" with the proper type (93)]
     expected: FAIL
 
+  [Window interface: calling showModalDialog(DOMString,any) on window with too few arguments must throw TypeError]
+    disabled:
+      if e10s: https://bugzilla.mozilla.org/show_bug.cgi?id=981796
+
   [Window interface: calling createImageBitmap(ImageBitmapSource,long,long,long,long) on window with too few arguments must throw TypeError]
     expected: FAIL