diff -r 000000000000 -r 6474c204b198 browser/components/sessionstore/test/browser_history_cap.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/sessionstore/test/browser_history_cap.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,125 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * This test ensures that the preferences (added in bug 943339) that control how
+ * many back and forward button session history entries we store work correctly.
+ *
+ * It adds a number of entries to the session history, restores them and checks
+ * that the restored state matches the preferences.
+ */
+
+add_task(function *test_history_cap() {
+  const baseURL = "http://example.com/browser_history_cap#"
+  const maxEntries  = 9; // The number of generated session history entries.
+  const middleEntry = 4; // The zero-based index of the middle entry.
+
+  const maxBack1 = 2; // The history cap settings used for the first test,
+  const maxFwd1 = 3;  // where maxBack1 + 1 + maxFwd1 < maxEntries.
+
+  const maxBack2 = 5; // The history cap settings used for the other tests, 
+  const maxFwd2 = 5;  // where maxBack2 + 1 + maxFwd2 > maxEntries.
+
+  // Set the relevant preferences for the first test.
+  gPrefService.setIntPref("browser.sessionhistory.max_entries", maxEntries);
+  gPrefService.setIntPref("browser.sessionstore.max_serialize_back", maxBack1);
+  gPrefService.setIntPref("browser.sessionstore.max_serialize_forward", maxFwd1);
+
+  // Make sure the settings we modify are reset afterward.
+  registerCleanupFunction(() => {
+    gPrefService.clearUserPref("browser.sessionhistory.max_entries");
+    gPrefService.clearUserPref("browser.sessionstore.max_serialize_back");
+    gPrefService.clearUserPref("browser.sessionstore.max_serialize_forward");
+  });
+
+  let tab = gBrowser.addTab();
+  let browser = tab.linkedBrowser;
+  yield promiseBrowserLoaded(browser);
+
+  // Generate the tab state entries and set the one-based
+  // tab-state index to the middle session history entry.
+  let tabState = {entries: [], index: middleEntry + 1};
+  for (let i = 0; i < maxEntries; i++) {
+    tabState.entries.push({url: baseURL + i});
+  }
+
+  info("Testing situation where only a subset of session history entries should be restored.");
+
+  ss.setTabState(tab, JSON.stringify(tabState));
+  yield promiseTabRestored(tab);
+  SyncHandlers.get(tab.linkedBrowser).flush();
+
+  let restoredTabState = JSON.parse(ss.getTabState(tab));
+  is(restoredTabState.entries.length, maxBack1 + 1 + maxFwd1,
+    "The expected number of session history entries was restored.");
+  is(restoredTabState.index, maxBack1 + 1, "The restored tab-state index is correct");
+
+  let indexURLOffset = middleEntry - (restoredTabState.index - 1);
+  for (let i = 0; i < restoredTabState.entries.length; i++) {
+    is(restoredTabState.entries[i].url, baseURL + (i + indexURLOffset),
+        "URL of restored entry matches the expected URL.");
+  }
+
+  // Set the relevant preferences for the other tests.
+  gPrefService.setIntPref("browser.sessionstore.max_serialize_back", maxBack2);
+  gPrefService.setIntPref("browser.sessionstore.max_serialize_forward", maxFwd2);
+
+  info("Testing situation where all of the entries in the session history should be restored.");
+
+  ss.setTabState(tab, JSON.stringify(tabState));
+  yield promiseTabRestored(tab);
+  SyncHandlers.get(tab.linkedBrowser).flush();
+
+  restoredTabState = JSON.parse(ss.getTabState(tab));
+  is(restoredTabState.entries.length, maxEntries,
+    "The expected number of session history entries was restored.");
+  is(restoredTabState.index, middleEntry + 1, "The restored tab-state index is correct");
+
+  for (let i = middleEntry - 2; i <= middleEntry + 2; i++) {
+    is(restoredTabState.entries[i].url, baseURL + i,
+        "URL of restored entry matches the expected URL.");
+  }
+
+  info("Testing situation where only the 1 + maxFwd2 oldest entries should be restored.");
+
+  // Set the one-based tab-state index to the oldest session history entry.
+  tabState.index = 1;
+
+  ss.setTabState(tab, JSON.stringify(tabState));
+  yield promiseTabRestored(tab);
+  SyncHandlers.get(tab.linkedBrowser).flush();
+
+  restoredTabState = JSON.parse(ss.getTabState(tab));
+  is(restoredTabState.entries.length, 1 + maxFwd2,
+    "The expected number of session history entries was restored.");
+  is(restoredTabState.index, 1, "The restored tab-state index is correct");
+
+  for (let i = 0; i <= 2; i++) {
+    is(restoredTabState.entries[i].url, baseURL + i,
+        "URL of restored entry matches the expected URL.");
+  }
+
+  info("Testing situation where only the maxBack2 + 1 newest entries should be restored.");
+
+  // Set the one-based tab-state index to the newest session history entry.
+  tabState.index = maxEntries;
+
+  ss.setTabState(tab, JSON.stringify(tabState));
+  yield promiseTabRestored(tab);
+  SyncHandlers.get(tab.linkedBrowser).flush();
+
+  restoredTabState = JSON.parse(ss.getTabState(tab));
+  is(restoredTabState.entries.length, maxBack2 + 1,
+    "The expected number of session history entries was restored.");
+  is(restoredTabState.index, maxBack2 + 1, "The restored tab-state index is correct");
+
+  indexURLOffset = (maxEntries - 1) - maxBack2;
+  for (let i = maxBack2 - 2; i <= maxBack2; i++) {
+    is(restoredTabState.entries[i].url, baseURL + (i + indexURLOffset),
+        "URL of restored entry matches the expected URL.");
+  }
+
+  gBrowser.removeTab(tab);
+});