diff --git a/dom/tests/mochitest/pointerlock/test_pointerlock-api.html b/dom/tests/mochitest/pointerlock/test_pointerlock-api.html
index fe78d8712f30..fd392fd29d43 100644
--- a/dom/tests/mochitest/pointerlock/test_pointerlock-api.html
+++ b/dom/tests/mochitest/pointerlock/test_pointerlock-api.html
@@ -67,6 +67,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=633602
         // in content/html/content/test/test_fullscreen-api.html, which uses the same pattern.
         const isWinXP = navigator.userAgent.indexOf("Windows NT 5.1") != -1;
         const isOSXLion = navigator.userAgent.indexOf("Mac OS X 10.7") != -1;
+        const isOSXMtnLion = navigator.userAgent.indexOf("Mac OS X 10.8") != -1;
 
         function finish() {
           SpecialPowers.clearUserPref("full-screen-api.enabled");
@@ -81,8 +82,8 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=633602
             finish();
             return;
           }
-          if (isOSXLion) {
-            todo(false, "Can't reliably run full-screen tests on OS X Lion, see bug 744125");
+          if (isOSXLion || isOSXMtnLion) {
+            todo(false, "Can't reliably run full-screen tests on OS X Lion or Mountain Lion, see bug 744125");
             finish();
             return;
           }