diff --git a/toolkit/mozapps/shared/FileUtils.jsm b/toolkit/mozapps/shared/FileUtils.jsm
index 2f7f5bcf59f8..27f3bd6e4170 100644
--- a/toolkit/mozapps/shared/FileUtils.jsm
+++ b/toolkit/mozapps/shared/FileUtils.jsm
@@ -21,6 +21,7 @@
 #
 # Contributor(s):
 #  Robert Strong <robert.bugzilla@gmail.com>
+#  Philipp von Weitershausen <philipp@weitershausen.de>
 #
 # 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
@@ -103,6 +104,22 @@ var FileUtils = {
     return dir;
   },
 
+  /**
+   * Opens a file output stream for writing.
+   * @param   file
+   *          The file to write to.
+   * @param   modeFlags
+   *          (optional) File open flags. Can be undefined.
+   * @returns nsIFileOutputStream to write to.
+   * @note The stream is initialized with the DEFER_OPEN behavior flag.
+   *       See nsIFileOutputStream.
+   */
+  openFileOutputStream: function FileUtils_openFileOutputStream(file, modeFlags) {
+    var fos = Cc["@mozilla.org/network/file-output-stream;1"].
+              createInstance(Ci.nsIFileOutputStream);
+    return this._initFileOutputStream(fos, file, modeFlags);
+  },
+
   /**
    * Opens a safe file output stream for writing.
    * @param   file
@@ -116,6 +133,10 @@ var FileUtils = {
   openSafeFileOutputStream: function FileUtils_openSafeFileOutputStream(file, modeFlags) {
     var fos = Cc["@mozilla.org/network/safe-file-output-stream;1"].
               createInstance(Ci.nsIFileOutputStream);
+    return this._initFileOutputStream(fos, file, modeFlags);
+  },
+
+ _initFileOutputStream: function FileUtils__initFileOutputStream(fos, file, modeFlags) {
     if (modeFlags === undefined)
       modeFlags = this.MODE_WRONLY | this.MODE_CREATE | this.MODE_TRUNCATE;
     fos.init(file, modeFlags, this.PERMS_FILE, fos.DEFER_OPEN);
diff --git a/toolkit/mozapps/shared/test/unit/test_FileUtils.js b/toolkit/mozapps/shared/test/unit/test_FileUtils.js
index 3e7bf8b67598..832ed84e305d 100644
--- a/toolkit/mozapps/shared/test/unit/test_FileUtils.js
+++ b/toolkit/mozapps/shared/test/unit/test_FileUtils.js
@@ -91,6 +91,44 @@ add_test(function test_getDir_shouldCreate() {
   run_next_test();
 });
 
+add_test(function test_openFileOutputStream_defaultFlags() {
+  let file = FileUtils.getFile("ProfD", ["george"]);
+  let fos = FileUtils.openFileOutputStream(file);
+  do_check_true(fos instanceof Components.interfaces.nsIFileOutputStream);
+
+  // FileUtils.openFileOutputStream() opens the stream with DEFER_OPEN
+  // which means the file will not be open until we write to it.
+  do_check_false(file.exists());
+
+  let data = "imagine";
+  fos.write(data, data.length);
+  do_check_true(file.exists());
+
+  // No nsIXULRuntime in xpcshell, so use this trick to determine whether we're
+  // on Windows.
+  if ("@mozilla.org/windows-registry-key;1" in Components.classes) {
+    do_check_eq(file.permissions, 0666);
+  } else {
+    do_check_eq(file.permissions, FileUtils.PERMS_FILE);
+  }
+
+  run_next_test();
+});
+
+// openFileOutputStream will uses MODE_WRONLY | MODE_CREATE | MODE_TRUNCATE
+// as the default mode flags, but we can pass in our own if we want to.
+add_test(function test_openFileOutputStream_modeFlags() {
+  let file = FileUtils.getFile("ProfD", ["ringo"]);
+  let fos = FileUtils.openFileOutputStream(file, FileUtils.MODE_WRONLY);
+  let data = "yesterday";
+  do_check_throws(function () {
+    fos.write(data, data.length);
+  }, Components.results.NS_ERROR_FILE_NOT_FOUND);
+  do_check_false(file.exists());
+
+  run_next_test();
+});
+
 add_test(function test_openSafeFileOutputStream_defaultFlags() {
   let file = FileUtils.getFile("ProfD", ["john"]);
   let fos = FileUtils.openSafeFileOutputStream(file);