diff --git a/toolkit/components/places/src/nsNavHistory.cpp b/toolkit/components/places/src/nsNavHistory.cpp
index 80eee56af1d6..498d8024aab1 100644
--- a/toolkit/components/places/src/nsNavHistory.cpp
+++ b/toolkit/components/places/src/nsNavHistory.cpp
@@ -384,6 +384,7 @@ nsNavHistory::nsNavHistory()
 , mBatchHasTransaction(PR_FALSE)
 , mCachedNow(0)
 , mExpireNowTimer(nsnull)
+, mLastSessionID(0)
 , mExpireDaysMin(0)
 , mExpireDaysMax(0)
 , mExpireSites(0)
@@ -470,25 +471,6 @@ nsNavHistory::Init()
   rv = NS_DispatchToMainThread(completeEvent);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  // extract the last session ID so we know where to pick up. There is no index
-  // over sessions so the naive statement "SELECT MAX(session) FROM
-  // moz_historyvisits" won't have good performance.
-  // This is long before we use our temporary tables, so we do not have to join
-  // on moz_historyvisits_temp to get the right result here.
-  {
-    nsCOMPtr<mozIStorageStatement> selectSession;
-    rv = mDBConn->CreateStatement(NS_LITERAL_CSTRING(
-        "SELECT session FROM moz_historyvisits "
-        "ORDER BY visit_date DESC LIMIT 1"),
-      getter_AddRefs(selectSession));
-    NS_ENSURE_SUCCESS(rv, rv);
-    PRBool hasSession;
-    if (NS_SUCCEEDED(selectSession->ExecuteStep(&hasSession)) && hasSession)
-      mLastSessionID = selectSession->AsInt64(0);
-    else
-      mLastSessionID = 1;
-  }
-
   // recent events hash tables
   NS_ENSURE_TRUE(mRecentTyped.Init(128), NS_ERROR_OUT_OF_MEMORY);
   NS_ENSURE_TRUE(mRecentBookmark.Init(128), NS_ERROR_OUT_OF_MEMORY);
@@ -2113,6 +2095,33 @@ nsNavHistory::LoadPrefs(PRBool aInitializing)
   return NS_OK;
 }
 
+
+PRInt64
+nsNavHistory::GetNewSessionID()
+{
+  // Use cached value if already initialized.
+  if (mLastSessionID)
+    return ++mLastSessionID;
+
+  // Extract the last session ID, so we know where to pick up. There is no
+  // index over sessions so we use the visit_date index.
+  // This happens on the first visit, so we don't care about temp tables.
+  nsCOMPtr<mozIStorageStatement> selectSession;
+  nsresult rv = mDBConn->CreateStatement(NS_LITERAL_CSTRING(
+      "SELECT session FROM moz_historyvisits "
+      "ORDER BY visit_date DESC LIMIT 1"),
+    getter_AddRefs(selectSession));
+  NS_ENSURE_SUCCESS(rv, rv);
+  PRBool hasSession;
+  if (NS_SUCCEEDED(selectSession->ExecuteStep(&hasSession)) && hasSession)
+    mLastSessionID = selectSession->AsInt64(0) + 1;
+  else
+    mLastSessionID = 1;
+
+  return mLastSessionID;
+}
+
+
 PRInt32
 nsNavHistory::GetDaysOfHistory() {
   PRInt32 daysOfHistory = 0;
diff --git a/toolkit/components/places/src/nsNavHistory.h b/toolkit/components/places/src/nsNavHistory.h
index fefafc98e12c..d8d227a03621 100644
--- a/toolkit/components/places/src/nsNavHistory.h
+++ b/toolkit/components/places/src/nsNavHistory.h
@@ -642,9 +642,9 @@ protected:
   PRBool GetRedirectFor(const nsACString& aDestination, nsACString& aSource,
                         PRTime* aTime, PRUint32* aRedirectType);
 
-  // session tracking
+  // Sessions tracking.
   PRInt64 mLastSessionID;
-  PRInt64 GetNewSessionID() { mLastSessionID ++; return mLastSessionID; }
+  PRInt64 GetNewSessionID();
 
 #ifdef MOZ_XUL
   // AutoComplete stuff
@@ -659,7 +659,7 @@ protected:
   PRInt32 mExpireDaysMax;
   PRInt32 mExpireSites;
 
-  // frecency prefs
+  // Frecency preferences.
   PRInt32 mNumVisitsForFrecency;
   PRInt32 mFirstBucketCutoffInDays;
   PRInt32 mSecondBucketCutoffInDays;