Difference Between Firefox and I.E

Hello,
 I'm having problems with some code in I:E which works fine in FireFox, what I want to do is if a user clicks on a page but doesnt have a cookie set up to redirect to a login page but keep a record of where they have come from so when the cookie is set up they can be redirected to where they started from.
Is there a better way of doing this or can someone see why this isnt working in I.E?  The problem I'm getting is that the session("location") is empty when I come to redirect back to where I came from.
Dim prot, https, domainname, filename, querystring As String

prot = "http"
https = LCase(Request.ServerVariables("HTTPS"))
If https <> "off" Then prot = "https"
d ...