FormsAuthenticationTicket issues in firefox(multiple instance of browser)

Hi All,
I am using forms authentication in asp.net application. This is working fine in internet explorer. Opening application in firefox is also fine but when i login in second instance of firefox, application in first instance logs out. Following is my code for creating ticket and cookie.
FormsAuthenticationTicket objTicket = new FormsAuthenticationTicket(1, sUserName, DateTime.Now, DateTime.Now.AddMinutes(60), fa ...