diff --git a/content/base/test/test_mutationobservers.html b/content/base/test/test_mutationobservers.html
index ffde7ce3113..88a4e143ec9 100644
--- a/content/base/test/test_mutationobservers.html
+++ b/content/base/test/test_mutationobservers.html
@@ -487,8 +487,12 @@ function testModalDialog() {
     });
   m.observe(div, { childList: true });
   div.innerHTML = "<span><span>foo</span></span>";
-  window.showModalDialog("mutationobserver_dialog.html");
-  ok(didHandleCallback, "Should have called the callback while showing modal dialog!");
+  try {
+    window.showModalDialog("mutationobserver_dialog.html");
+    ok(didHandleCallback, "Should have called the callback while showing modal dialog!");
+  } catch(e) {
+    todo(false, "showModalDialog not implemented on this platform");
+  }
   then(testTakeRecords);
 }
 
diff --git a/testing/mochitest/android.json b/testing/mochitest/android.json
index 342bf469add..b7dffde5156 100644
--- a/testing/mochitest/android.json
+++ b/testing/mochitest/android.json
@@ -23,7 +23,6 @@
  "content/base/test/test_mixed_content_blocker.html": "TIMED_OUT, SSL_REQUIRED",
  "content/base/test/test_mixed_content_blocker_bug803225.html": "TIMED_OUT, SSL_REQUIRED",
  "content/base/test/test_mixed_content_blocker_frameNavigation.html": "TIMED_OUT, SSL_REQUIRED",
- "content/base/test/test_mutationobservers.html": "",
  "content/base/test/test_plugin_freezing.html": "CLICK_TO_PLAY",
  "content/base/test/test_object.html": "",
  "content/base/test/test_range_bounds.html": "",
diff --git a/testing/mochitest/androidx86.json b/testing/mochitest/androidx86.json
index e140142023b..2773b58e74a 100644
--- a/testing/mochitest/androidx86.json
+++ b/testing/mochitest/androidx86.json
@@ -21,7 +21,6 @@
  "content/base/test/test_mixed_content_blocker.html": "TIMED_OUT, SSL_REQUIRED",
  "content/base/test/test_mixed_content_blocker_bug803225.html": "TIMED_OUT, SSL_REQUIRED",
  "content/base/test/test_mixed_content_blocker_frameNavigation.html": "TIMED_OUT, SSL_REQUIRED",
- "content/base/test/test_mutationobservers.html": "",
  "content/base/test/test_plugin_freezing.html": "CLICK_TO_PLAY",
  "content/base/test/test_object.html": "",
  "content/base/test/test_range_bounds.html": "",
diff --git a/testing/mochitest/b2g.json b/testing/mochitest/b2g.json
index 3643e5a9749..6ac36f10a28 100644
--- a/testing/mochitest/b2g.json
+++ b/testing/mochitest/b2g.json
@@ -82,7 +82,6 @@
     "content/base/test/test_bug666604.html":"",
     "content/base/test/test_bug675121.html":"",
     "content/base/test/test_classList.html":"",
-    "content/base/test/test_mutationobservers.html":"",
     "content/base/test/test_title.html":"",
     "content/canvas/test/crossorigin/test_video_crossorigin.html":"",
     "content/events/test/test_bug422132.html":"",