diff -r 000000000000 -r 6474c204b198 browser/base/content/test/general/browser_tabs_isActive.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/base/content/test/general/browser_tabs_isActive.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,29 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+function test() {
+  test_tab("about:blank");
+  test_tab("about:license");
+}
+
+function test_tab(url) {
+  let originalTab = gBrowser.selectedTab;
+  let newTab = gBrowser.addTab(url, {skipAnimation: true});
+  is(tabIsActive(newTab), false, "newly added " + url + " tab is not active");
+  is(tabIsActive(originalTab), true, "original tab is active initially");
+
+  gBrowser.selectedTab = newTab;
+  is(tabIsActive(newTab), true, "newly added " + url + " tab is active after selection");
+  is(tabIsActive(originalTab), false, "original tab is not active while unselected");
+
+  gBrowser.selectedTab = originalTab;
+  is(tabIsActive(newTab), false, "newly added " + url + " tab is not active after switch back");
+  is(tabIsActive(originalTab), true, "original tab is active again after switch back");
+  
+  gBrowser.removeTab(newTab);
+}
+
+function tabIsActive(tab) {
+  let browser = tab.linkedBrowser;
+  return browser.docShell.isActive;
+}