Add this to your script right under your session_start(); call.
header("Cache-control: private");
I am not 100% certain what this does, other than to say I was told it was a fix for IE6 and Mozilla regarding sessions. Maybe one of the Session/Header gurus out there can shed some light on this. I may be way out in left field, but like I said, this was told to me once, and I put it in my snippet library.