Are you running any software firewall, internet security suite, or anti-virus app with integrated firewall/security features?
Disable them temporarily, and see if it works.
Although you say that the same happens not only to FF but also to IE, it may be still worth trying with a new clean FF profile.
Learn here how to proceed:
http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows