diff -r 000000000000 -r 6474c204b198 browser/devtools/netmonitor/test/browser_net_html-preview.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/devtools/netmonitor/test/browser_net_html-preview.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,62 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Tests if html responses show and properly populate a "Preview" tab.
+ */
+
+function test() {
+  initNetMonitor(CONTENT_TYPE_URL).then(([aTab, aDebuggee, aMonitor]) => {
+    info("Starting test... ");
+
+    let { $, document, EVENTS, NetMonitorView } = aMonitor.panelWin;
+    let { RequestsMenu } = NetMonitorView;
+
+    RequestsMenu.lazyUpdate = false;
+
+    waitForNetworkEvents(aMonitor, 6).then(() => {
+      EventUtils.sendMouseEvent({ type: "mousedown" },
+        document.getElementById("details-pane-toggle"));
+
+      is($("#event-details-pane").selectedIndex, 0,
+        "The first tab in the details pane should be selected.");
+      is($("#preview-tab").hidden, true,
+        "The preview tab should be hidden for non html responses.");
+      is($("#preview-tabpanel").hidden, true,
+        "The preview tabpanel should be hidden for non html responses.");
+
+      RequestsMenu.selectedIndex = 4;
+      NetMonitorView.toggleDetailsPane({ visible: true, animated: false }, 5);
+
+      is($("#event-details-pane").selectedIndex, 5,
+        "The fifth tab in the details pane should be selected.");
+      is($("#preview-tab").hidden, false,
+        "The preview tab should be visible now.");
+      is($("#preview-tabpanel").hidden, false,
+        "The preview tabpanel should be visible now.");
+
+      waitFor(aMonitor.panelWin, EVENTS.RESPONSE_HTML_PREVIEW_DISPLAYED).then(() => {
+        let iframe = $("#response-preview");
+        ok(iframe,
+          "There should be a response preview iframe available.");
+        ok(iframe.contentDocument,
+          "The iframe's content document should be available.");
+        is(iframe.contentDocument.querySelector("blink").textContent, "Not Found",
+          "The iframe's content document should be loaded and correct.");
+
+        RequestsMenu.selectedIndex = 5;
+
+        is($("#event-details-pane").selectedIndex, 0,
+          "The first tab in the details pane should be selected again.");
+        is($("#preview-tab").hidden, true,
+          "The preview tab should be hidden again for non html responses.");
+        is($("#preview-tabpanel").hidden, true,
+          "The preview tabpanel should be hidden again for non html responses.");
+
+        teardown(aMonitor).then(finish);
+      });
+    });
+
+    aDebuggee.performRequests();
+  });
+}