Well, to be more specific, and after some testing, it works fine on FF 3.0.3 but doesn´t work on newest version of FF - 3.0.4. ...

Hi I have the same issue! After logging in , its redirecting me to xml feed! It´s happenig in FF 3.04 , in IE it is ok.
I have the very last version of punbb 1.3 .
Do you have some idea? thx