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diff --git a/dom/heapsnapshot/tests/unit/test_DominatorTree_05.js b/dom/heapsnapshot/tests/unit/test_DominatorTree_05.js
new file mode 100644
index 000000000..c07cee994
--- /dev/null
+++ b/dom/heapsnapshot/tests/unit/test_DominatorTree_05.js
@@ -0,0 +1,75 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// Test that we can get the set of immediately dominated nodes for any given
+// node and that this forms a tree.
+
+function run_test() {
+  var dominatorTree = saveHeapSnapshotAndComputeDominatorTree();
+  equal(typeof dominatorTree.getImmediatelyDominated, "function",
+        "getImmediatelyDominated should be a function");
+
+  // Do a traversal of the dominator tree.
+  //
+  // Note that we don't assert directly, only if we get an unexpected
+  // value. There are just way too many nodes in the heap graph to assert for
+  // every one. This test would constantly time out and assertion messages would
+  // overflow the log size.
+
+  var root = dominatorTree.root;
+  equal(dominatorTree.getImmediateDominator(root), null,
+        "The root should not have a parent");
+
+  var seen = new Set();
+  var stack = [root];
+  while (stack.length > 0) {
+    var top = stack.pop();
+
+    if (seen.has(top)) {
+      ok(false,
+         "This is a tree, not a graph: we shouldn't have multiple edges to the same node");
+    }
+    seen.add(top);
+    if (seen.size % 1000 === 0) {
+      dumpn("Progress update: seen size = " + seen.size);
+    }
+
+    var newNodes = dominatorTree.getImmediatelyDominated(top);
+    if (Object.prototype.toString.call(newNodes) !== "[object Array]") {
+      ok(false, "getImmediatelyDominated should return an array for known node ids");
+    }
+
+    var topSize = dominatorTree.getRetainedSize(top);
+
+    var lastSize = Infinity;
+    for (var i = 0; i < newNodes.length; i++) {
+      if (typeof newNodes[i] !== "number") {
+        ok(false, "Every dominated id should be a number");
+      }
+
+      if (dominatorTree.getImmediateDominator(newNodes[i]) !== top) {
+        ok(false, "child's parent should be the expected parent");
+      }
+
+      var thisSize = dominatorTree.getRetainedSize(newNodes[i]);
+
+      if (thisSize >= topSize) {
+        ok(false, "the size of children in the dominator tree should always be less than that of their parent");
+      }
+
+      if (thisSize > lastSize) {
+        ok(false,
+           "children should be sorted by greatest to least retained size, "
+           + "lastSize = " + lastSize + ", thisSize = " + thisSize);
+      }
+
+      lastSize = thisSize;
+      stack.push(newNodes[i]);
+    }
+  }
+
+  ok(true, "Successfully walked the tree");
+  dumpn("Walked " + seen.size + " nodes");
+
+  do_test_finished();
+}