[py] Add to Firefox driver too

Author: cgoldberg

py/selenium/webdriver/firefox/webdriver.py

@@ -226,3 +226,6 @@ def download_file(self, *args, **kwargs):
 
     def get_downloadable_files(self, *args, **kwargs):
         raise NotImplementedError
+
+    def delete_downloadable_files(self, *args, **kwargs):
+        raise NotImplementedError