Penetration Testing: Odd XSS finding

["guiness.stout" <guinness.stout () gmail com>]

I was hoping someone could shed some light on this odd XSS
vulnerability I uncovered while doing a pentest for a client.  The
site is a customer portal and when the below XSS is executed nothing
happens.  Basically gives a session error back, nothing interesting
there.  But when you kill -9 or End Process on FireFox then reopen
with "Restore Session" the site comes back up to the XSS but dumps
logged in users information.

I cannot replicate this in other browsers nor with Paros, webscarab, SPIKE etc.

https://host/portal/j_acegi_security_check?j_username=%27%3Cscript%3Ealert%28%27xss%27%29%3C%2Fscript%3E&j_password=d&login=Login

-Chris

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