Update firefox_profile.py

Fixes #1294

Signed-off-by: Luke Inman-Semerau <luke.semerau@gmail.com>

Author: erikbeerepoot

py/selenium/webdriver/firefox/firefox_profile.py

@@ -164,6 +164,7 @@ def encoded(self):
         A zipped, base64 encoded string of profile directory
         for use with remote WebDriver JSON wire protocol
         """
+        self.update_preferences()
         fp = BytesIO()
         zipped = zipfile.ZipFile(fp, 'w', zipfile.ZIP_DEFLATED)
         path_root = len(self.path) + 1  # account for trailing slash