diff -r 000000000000 -r 6474c204b198 browser/base/content/test/general/browser_visibleLabel.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/base/content/test/general/browser_visibleLabel.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,94 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/* Tests:
+ *   verify that the visibleLabel attribute works
+ *   verify the TabLabelModified event works for both existing and new tabs
+ */
+
+function test() {
+  waitForExplicitFinish();
+  registerCleanupFunction(function() {
+    gBrowser.removeCurrentTab({animate: false});
+  });
+  let tab = gBrowser.selectedTab = gBrowser.addTab("about:blank",
+                                                   {skipAnimation: true});
+  tab.linkedBrowser.addEventListener("load", function onLoad(event) {
+    event.currentTarget.removeEventListener("load", onLoad, true);
+    executeSoon(afterLoad);
+  }, true);
+}
+
+function afterLoad() {
+  let tab = gBrowser.selectedTab;
+  let xulLabel = document.getAnonymousElementByAttribute(tab, "anonid",
+                                                         "tab-label");
+  // Verify we're starting out on the right foot
+  is(tab.label, "New Tab", "Initial tab label is default");
+  is(xulLabel.value, "New Tab", "Label element is default");
+  is(tab.visibleLabel, "New Tab", "visibleLabel is default");
+
+  // Check that a normal label setting works correctly
+  tab.label = "Hello, world!";
+  is(tab.label, "Hello, world!", "tab label attribute set via tab.label");
+  is(xulLabel.value, "Hello, world!", "xul:label set via tab.label");
+  is(tab.visibleLabel, "Hello, world!", "visibleLabel set via tab.label");
+
+  // Check that setting visibleLabel only affects the label element
+  tab.visibleLabel = "Goodnight, Irene";
+  is(tab.label, "Hello, world!", "Tab.label unaffected by visibleLabel setter");
+  is(xulLabel.value, "Goodnight, Irene",
+     "xul:label set by visibleLabel setter");
+  is(tab.visibleLabel, "Goodnight, Irene",
+     "visibleLabel attribute set by visibleLabel setter");
+
+  // Check that setting the label property hits everything
+  tab.label = "One more label";
+  is(tab.label, "One more label",
+     "Tab label set via label property after diverging from visibleLabel");
+  is(xulLabel.value, "One more label",
+     "xul:label set via label property after diverging from visibleLabel");
+  is(tab.visibleLabel, "One more label",
+     "visibleLabel set from label property after diverging from visibleLabel");
+
+  tab.addEventListener("TabLabelModified", overrideTabLabel, true);
+  tab.label = "This won't be the visibleLabel";
+}
+
+function overrideTabLabel(aEvent) {
+  aEvent.target.removeEventListener("TabLabelModified", overrideTabLabel, true);
+  aEvent.preventDefault();
+  aEvent.stopPropagation();
+  aEvent.target.visibleLabel = "Handler set this as the visible label";
+  executeSoon(checkTabLabelModified);
+}
+
+function checkTabLabelModified() {
+  let tab = gBrowser.selectedTab;
+  let xulLabel = document.getAnonymousElementByAttribute(tab, "anonid",
+                                                         "tab-label");
+
+  is(tab.label, "This won't be the visibleLabel",
+     "Tab label set via label property that triggered event");
+  is(xulLabel.value, "Handler set this as the visible label",
+     "xul:label set by TabLabelModified handler");
+  is(tab.visibleLabel, "Handler set this as the visible label",
+     "visibleLabel set by TabLabelModified handler");
+
+  gBrowser.removeCurrentTab({animate: false});
+  executeSoon(checkTabLabelModifiedOnNewTab);
+}
+
+function checkTabLabelModifiedOnNewTab() {
+  gBrowser.tabContainer.addEventListener("TabLabelModified",
+    handleTabLabelModifiedOnNewTab, true);
+  let tab = gBrowser.selectedTab = gBrowser.addTab("about:blank",
+                                                   {skipAnimation: true});
+}
+
+function handleTabLabelModifiedOnNewTab(aEvent) {
+  gBrowser.tabContainer.removeEventListener("TabLabelModified",
+    handleTabLabelModifiedOnNewTab, true);
+  ok(true, "Event received from new tab default being set");
+  executeSoon(finish);
+}