When i login with user A login credentials in one web browser(chrome) and run my application,and i login with same user in another browser(Firefox)
That's two different sessions that are independent of each other.
how to automatically previous web browser session expire using form authentication in asp.net
I don't have a clue as to what you are asking, because your sentence is badly formed that makes no sense.
Anyway, ASP.NET issues are discussed at the ASP.NET forum.
http://forums.asp.net/