Some Sites doesn't be loaded

Hi,

i had some trouble with my FF Portable.

FF runs on an USB Stick behind a Proxy and Firewall. My Networksetting are over a automatic Proxyconfiguration.

Some site, e.g. heise.de mail.google.com and so on, dosen't be loaded. Over the local IE does it. The Proxy is for IE the same. So the Proxy isn't the problem.

Anybody an idea whats the problem is?

I did not find any similarities of the pages wich dind't be show.

Thanks and sorry for the bad english

EDIT:
Ok, i belive the poblem is the cookies. I don't no why but i delete the cookie of heise.de and now i can access the site. On my Blog site it dosen't work at the moment but i try it.