--- a/modules/libpref/src/prefapi.cpp
+++ b/modules/libpref/src/prefapi.cpp
@@ -327,18 +327,16 @@ pref_savePref(PLDHashTable *table, PLDHa
     // where we're getting our pref from
     PrefValue* sourcePref;
 
     if (PREF_HAS_USER_VALUE(pref) &&
         pref_ValueChanged(pref->defaultPref,
                           pref->userPref,
                           (PrefType) PREF_TYPE(pref)))
         sourcePref = &pref->userPref;
-    else if (PREF_IS_LOCKED(pref))
-        sourcePref = &pref->defaultPref;
     else
         // do not save default prefs that haven't changed
         return PL_DHASH_NEXT;
 
     // strings are in quotes!
     if (pref->flags & PREF_STRING) {
         prefValue = '\"';
         str_escape(sourcePref->stringVal, prefValue);

new file mode 100644
--- /dev/null
+++ b/modules/libpref/test/unit/test_bug506224.js
@@ -0,0 +1,29 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/licenses/publicdomain/  */
+
+function run_test() {
+  const Cc = Components.classes;
+  const Ci = Components.interfaces;
+  const PREF_NAME = "testPref";
+
+  var ps = Cc["@mozilla.org/preferences-service;1"]
+           .getService(Ci.nsIPrefService);
+  var prefs = ps.getDefaultBranch(null);
+  var userprefs = ps.getBranch(null);
+
+  prefs.setCharPref(PREF_NAME, "test0");
+  prefs.lockPref(PREF_NAME);
+  do_check_eq("test0", userprefs.getCharPref(PREF_NAME));
+  do_check_eq(false, userprefs.prefHasUserValue(PREF_NAME));
+
+  var file = do_get_profile();
+  file.append("prefs.js");
+  ps.savePrefFile(file);
+
+  prefs.unlockPref(PREF_NAME);
+  prefs.setCharPref(PREF_NAME, "test1");
+  ps.readUserPrefs(file);
+
+  do_check_eq("test1", userprefs.getCharPref(PREF_NAME));
+  do_check_eq(false, userprefs.prefHasUserValue(PREF_NAME));
+}