diff --git a/xpcom/ds/Makefile.in b/xpcom/ds/Makefile.in
index 584b3b7b2fb..09d6b5bdc8f 100644
--- a/xpcom/ds/Makefile.in
+++ b/xpcom/ds/Makefile.in
@@ -90,6 +90,7 @@ EXPORTS_NAMESPACES = mozilla
 EXPORTS_mozilla = \
   CharTokenizer.h \
   TimeStamp.h \
+  StringBuilder.h \
   $(NULL)
 
 EXPORTS		= \
diff --git a/xpcom/ds/StringBuilder.h b/xpcom/ds/StringBuilder.h
new file mode 100644
index 00000000000..5fcd478dcf4
--- /dev/null
+++ b/xpcom/ds/StringBuilder.h
@@ -0,0 +1,116 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* vim:set ts=4 sw=4 sts=4 et cindent: */
+/* ***** BEGIN LICENSE BLOCK *****
+ * Version: MPL 1.1/GPL 2.0/LGPL 2.1
+ *
+ * The contents of this file are subject to the Mozilla Public License Version
+ * 1.1 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ * http://www.mozilla.org/MPL/
+ *
+ * Software distributed under the License is distributed on an "AS IS" basis,
+ * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
+ * for the specific language governing rights and limitations under the
+ * License.
+ *
+ * The Original Code is Mozilla code.
+ *
+ * The Initial Developer of the Original Code is the Mozilla Foundation.
+ * Portions created by the Initial Developer are Copyright (C) 2011
+ * the Initial Developer. All Rights Reserved.
+ *
+ * Contributor(s):
+ *  Jeff Muizelaar <jmuizelaar@mozilla.com>
+ *
+ * Alternatively, the contents of this file may be used under the terms of
+ * either the GNU General Public License Version 2 or later (the "GPL"), or
+ * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+ * in which case the provisions of the GPL or the LGPL are applicable instead
+ * of those above. If you wish to allow use of your version of this file only
+ * under the terms of either the GPL or the LGPL, and not to allow others to
+ * use your version of this file under the terms of the MPL, indicate your
+ * decision by deleting the provisions above and replace them with the notice
+ * and other provisions required by the GPL or the LGPL. If you do not delete
+ * the provisions above, a recipient may use your version of this file under
+ * the terms of any one of the MPL, the GPL or the LGPL.
+ *
+ * ***** END LICENSE BLOCK ***** */
+
+/* We would use std::max but MS makes it painful
+// windef.h defines min and max macros that we don't want
+// http://support.microsoft.com/kb/143208
+#ifdef _WIN32
+#define NOMINMAX
+#endif
+*/
+
+#include <stdlib.h>
+#include <string.h>
+#include "nsAlgorithm.h"
+
+/* This is a standard string builder like ones in Java
+   or C#. It uses a doubling allocation strategy
+   to grow when out of capacity.
+
+   This does not use nsTArray because nsTArray starts
+   growing by multiples of page size after it is the
+   size of one page. We want to keep doubling in size
+   so that we can continue to append at high speed even
+   for large strings.
+
+   Eventually, this should be templated for wide characters.
+
+ */
+
+namespace mozilla {
+
+class StringBuilder
+{
+public:
+    StringBuilder() {
+        mCapacity = 16;
+        mLength = 0;
+        mBuffer = static_cast<char*>(malloc(sizeof(char)*mCapacity));
+        mBuffer[0] = '\0';
+    }
+
+    void Append(const char *s) {
+        size_t newLength = strlen(s);
+
+        EnsureCapacity(mLength + newLength+1);
+
+        // copy the entire string including the null terminator
+        memcpy(&mBuffer[mLength], s, newLength+1);
+        mLength += newLength;
+    }
+
+    char *Buffer() {
+        return mBuffer;
+    }
+
+    size_t Length() {
+        return mLength;
+    }
+
+    size_t EnsureCapacity(size_t capacity) {
+        if (capacity > mCapacity) {
+            // make sure we at least double in size
+            mCapacity = NS_MAX(capacity, mCapacity*2);
+            mBuffer = static_cast<char*>(realloc(mBuffer, mCapacity));
+            mCapacity = moz_malloc_usable_size(mBuffer);
+        }
+        return mCapacity;
+    }
+
+    ~StringBuilder()
+    {
+        free(mBuffer);
+    }
+
+private:
+    char *mBuffer;
+    size_t mLength; // the length of the contained string not including the null terminator
+    size_t mCapacity; // the total size of mBuffer
+};
+
+}