Synchronize firefox webextension code

firefox_webextension/compat.js

@@ -17,6 +17,11 @@
   along with gPass.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+var default_preferences = {"pbkdf2_level": 1000,
+			   "account_url": "https://gpass-demo.soutade.fr/demo",
+                           "always_disabled":false
+			  };
+
 function get_preference(key)
 {
     return browser.storage.local.get(key)
@@ -38,9 +43,11 @@ function get_preference(key)
 	);
 }
 
-function set_preference(key, value)
+function set_preference(key, value, sendResponse)
 {
     browser.storage.local.set({[key]:value});
+    if (sendResponse)
+        sendResponse(true);
 }
 
 function delete_preference(key)