--- a/dom/base/test/test_fileapi_slice.html
+++ b/dom/base/test/test_fileapi_slice.html
@@ -17,21 +17,16 @@ https://bugzilla.mozilla.org/show_bug.cg
   <canvas id=testcanvas hidden moz-opaque></canvas>
   <input id="fileList" type="file"></input>
 </p>
 <div id="content" style="display: none">
 </div>
 
 <pre id="test">
 <script class="testbody" type="text/javascript">
-const isOSXMtnLion = navigator.userAgent.indexOf("Mac OS X 10.8") != -1;
-
-if (isOSXMtnLion) {
-  todo(false, "Mountain Lion doesn't like this test (bug 788999)");
-} else {
 var fileNum = 1;
 SimpleTest.waitForExplicitFinish();
 
 // Create files containing data we'll test with. We'll want long
 // strings to ensure they span multiple buffers while loading
 
 // Create a decent-sized image
 cx = $("canvas").getContext('2d');
@@ -131,12 +126,11 @@ expectedTestCount++;
 
 // image past end
 var imgfile = createFileWithData(testBinaryData + fileData);
 is(imgfile.size, size + testBinaryData.length, "correct file size (past end)");
 var img = new Image;
 img.src = URL.createObjectURL(imgfile.slice(testBinaryData.length, testBinaryData.length + size + 1000));
 img.onload = imageLoadHandler;
 expectedTestCount++;
-}
 </script>
 </pre>
 </body> </html>