login problems from within FF

I am running portable ff off a thumbdrive at work. Whenever I try to login to certain sites (yahoo for email, msn for hotmail, espn insider) it times out. This happens with both stored passwords and newly typed. I have already cleared the saved passwords, the cache and the cookies. I have limited user rights on the machine, but ff works fine otherwise, and connects everywhere else. We are behind a firewall, but I can login to all the sites on IE. Any ideas why this is happening? Any fixes out there?

thanks
Vic