Heya,
Have you tried Ziphone? It can set your iphone into DFU mode by one click.
A very detailed instruction on how to get into DFU mode manually is here:
http://www.modmyi.com/forums/1976661-post2.html
It's good to hear I'm not the only one who can't connect to the apple update servers. But I never checked the firewall, maybe that'll help.
Alex.