diff -r 000000000000 -r 6474c204b198 browser/components/sessionstore/test/browser_swapDocShells.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/sessionstore/test/browser_swapDocShells.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,39 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+function test() {
+  waitForExplicitFinish();
+
+  Task.spawn(function task() {
+    let tab = gBrowser.selectedTab = gBrowser.addTab("about:mozilla");
+    yield promiseBrowserLoaded(gBrowser.selectedBrowser);
+
+    let win = gBrowser.replaceTabWithWindow(tab);
+    yield promiseDelayedStartupFinished(win);
+    yield promiseBrowserHasURL(win.gBrowser.browsers[0], "about:mozilla");
+
+    win.duplicateTabIn(win.gBrowser.selectedTab, "tab");
+    let browser = win.gBrowser.browsers[1];
+    yield promiseBrowserLoaded(browser);
+    is(browser.currentURI.spec, "about:mozilla", "tab was duplicated");
+
+    win.close();
+  }).then(finish);
+}
+
+function promiseDelayedStartupFinished(win) {
+  let deferred = Promise.defer();
+  whenDelayedStartupFinished(win, deferred.resolve);
+  return deferred.promise;
+}
+
+function promiseBrowserHasURL(browser, url) {
+  let promise = Promise.resolve();
+
+  if (browser.contentDocument.readyState === "complete" &&
+      browser.currentURI.spec === url) {
+    return promise;
+  }
+
+  return promise.then(() => promiseBrowserHasURL(browser, url));
+}