Firefox Issue for trueclient 12.53 web

Hi,

i am facing issue while running true client script in controller.

Load Runner 12.53 trial

Windows 7 SP1

Firefox 40

one scenarion with 20 users 15 min

when we start the execution for 20 users the transaction are passing as soon as the users are in running state but once all users are ramped up the transactions are stoped passing and we cant see anything until the users are ramped down and all the 20 users have same error message

Proxy Authentication Required window appears due to which we can not proceed with the execution

so how to handle this window in controller?

Note: we are getting this window in VUgen  while reply in load and interactive mode.

Thanks

Ashish Kate