Thanks Ankit for your reply !
In case of Firefox, if we open two browser windows simultaneously and sign in to the application in one window, the cookies are getting shared with the other window also. so closing the particular window (with 'x') alone will not delete the JSESSIONID and so the problem persists as the cookie is alive in the other window
In case of IE, I meant when we close the Tab, the problem occurs.
And It would be really helpful if you could elaborate why token session will not be a good hand here?.
And Is there any other way to solve this other than using onUnload to capture the close event as it has its own set of drawbacks.??