diff -r 000000000000 -r 6474c204b198 browser/devtools/webconsole/test/browser_longstring_hang.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/devtools/webconsole/test/browser_longstring_hang.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,64 @@
+/* vim:set ts=2 sw=2 sts=2 et: */
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// Test that very long strings do not hang the browser.
+
+function test()
+{
+  addTab("http://example.com/browser/browser/devtools/webconsole/test/test-bug-859170-longstring-hang.html");
+
+  let hud = null;
+
+  gBrowser.selectedBrowser.addEventListener("load", function onLoad() {
+    gBrowser.selectedBrowser.removeEventListener("load", onLoad, true);
+    openConsole(null, performTest);
+  }, true);
+
+  function performTest(aHud)
+  {
+    hud = aHud;
+    info("wait for the initial long string");
+
+    waitForMessages({
+      webconsole: hud,
+      messages: [
+        {
+          name: "find 'foobar', no 'foobaz', in long string output",
+          text: "foobar",
+          noText: "foobaz",
+          category: CATEGORY_WEBDEV,
+          longString: true,
+        },
+      ],
+    }).then(onInitialString);
+  }
+
+  function onInitialString(aResults)
+  {
+    let clickable = aResults[0].longStrings[0];
+    ok(clickable, "long string ellipsis is shown");
+    clickable.scrollIntoView(false);
+
+    EventUtils.synthesizeMouse(clickable, 2, 2, {}, hud.iframeWindow);
+
+    info("wait for long string expansion");
+
+    waitForMessages({
+      webconsole: hud,
+      messages: [
+        {
+          name: "find 'foobaz' after expand, but no 'boom!' at the end",
+          text: "foobaz",
+          noText: "boom!",
+          category: CATEGORY_WEBDEV,
+          longString: false,
+        },
+        {
+          text: "too long to be displayed",
+          longString: false,
+        },
+      ],
+    }).then(finishTest);
+  }
+}