diff -r 000000000000 -r 6474c204b198 browser/components/sessionstore/test/browser_687710.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/sessionstore/test/browser_687710.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,44 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// Test that sessionrestore handles cycles in the shentry graph properly.
+//
+// These cycles shouldn't be there in the first place, but they cause hangs
+// when they mysteriously appear (bug 687710).  Docshell code assumes this
+// graph is a tree and tires to walk to the root.  But if there's a cycle,
+// there is no root, and we loop forever.
+
+let stateBackup = ss.getBrowserState();
+
+let state = {windows:[{tabs:[{entries:[
+  {
+    docIdentifier: 1,
+    url: "http://example.com",
+    children: [
+      {
+        docIdentifier: 2,
+        url: "http://example.com"
+      }
+    ]
+  },
+  {
+    docIdentifier: 2,
+    url: "http://example.com",
+    children: [
+      {
+        docIdentifier: 1,
+        url: "http://example.com"
+      }
+    ]
+  }
+]}]}]}
+
+function test() {
+  registerCleanupFunction(function () {
+    ss.setBrowserState(stateBackup);
+  });
+
+  /* This test fails by hanging. */
+  ss.setBrowserState(JSON.stringify(state));
+  ok(true, "Didn't hang!");
+}