getting exceptions in OIM logs

Hi we have OIM server running in two nodes cluster. noticed following exceptions in logs when user logged in to /identity using single sign-on URL, page also become unresponsive. surprisingly this is only happening when using Firefox. with IE or chrome everything seems to work fine. any idea what error is about?

####<Aug 2, 2016 1:05:02 PM EDT> <Error> <oracle.adfinternal.view.rich.marshal.DefaultMarshalingService> <server1.corp.co> <oim_server1> <[ACTIVE] ExecuteThread: '0' for queue: 'weblogic.kernel.Default (self-tuning)'> <xelsysadm> <> <005EHaryPha0vl85rj9DiY0002iW00005J> <1470157502125> <BEA-000000> <

oracle.adfinternal.view.rich.marshal.MarshalerNotFoundException: No Encoder Found for namespace:https://oracle.com/richClient/comm;