diff -r 000000000000 -r 6474c204b198 testing/mochitest/tests/SimpleTest/WindowSnapshot.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/testing/mochitest/tests/SimpleTest/WindowSnapshot.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,63 @@
+var gWindowUtils;
+
+try {
+  gWindowUtils = SpecialPowers.getDOMWindowUtils(window);
+  if (gWindowUtils && !gWindowUtils.compareCanvases)
+    gWindowUtils = null;
+} catch (e) {
+  gWindowUtils = null;
+}
+
+function snapshotWindow(win, withCaret) {
+  return SpecialPowers.snapshotWindow(win, withCaret);
+}
+
+// If the two snapshots don't compare as expected (true for equal, false for
+// unequal), returns their serializations as data URIs.  In all cases, returns
+// whether the comparison was as expected.
+function compareSnapshots(s1, s2, expected) {
+  var s1Str, s2Str;
+  var correct = false;
+  if (gWindowUtils) {
+    // First, check that the canvases are the same size.
+    var equal;
+    if (s1.width != s2.width || s1.height != s2.height) {
+      equal = false;
+    } else {
+      try {
+        equal = (gWindowUtils.compareCanvases(s1, s2, {}) == 0);
+      } catch (e) {
+        equal = false;
+        ok(false, "Exception thrown from compareCanvases: " + e);
+      }
+    }
+    correct = (equal == expected);
+  }
+
+  if (!correct) {
+    s1Str = s1.toDataURL();
+    s2Str = s2.toDataURL();
+
+    if (!gWindowUtils) {
+	correct = ((s1Str == s2Str) == expected);
+    }
+  }
+
+  return [correct, s1Str, s2Str];
+}
+
+function assertSnapshots(s1, s2, expected, s1name, s2name) {
+  var [correct, s1Str, s2Str] = compareSnapshots(s1, s2, expected);
+  var sym = expected ? "==" : "!=";
+  ok(correct, "reftest comparison: " + sym + " " + s1name + " " + s2name);
+  if (!correct) {
+    var report = "REFTEST TEST-UNEXPECTED-FAIL | " + s1name + " | image comparison (" + sym + ")\n";
+    if (expected) {
+      report += "REFTEST   IMAGE 1 (TEST): " + s1Str + "\n";
+      report += "REFTEST   IMAGE 2 (REFERENCE): " + s2Str + "\n";
+    } else {
+      report += "REFTEST   IMAGE: " + s1Str + "\n";
+    }
+    dump(report);
+  }
+}