So let me get this right
1. You should connect your iPhone to iTunes first and then click on serial which will then show you the UID which u should copy.
2. Now sign up to be an apple developer and pay the $99.
3. Wait for the confirmation to be emailed to u. Some say 24hr. Some say 6 to 7 days.
4. Login to the portal and add your uid from just now there
5. Download the packaged file and extract the ipsw. Easily done with poweriso.
6. Connect iPhone to iTunes, shift+restore, locate the ipsw and all is fine
I don't see why u can't do it in windows
I did all thing in this order (only took 45 minutes in my case to get the confirmation).
But I forgot to register the UDID before doing all this WHICH IS THE MAJOR MISTAKE HERE. YEAH-Stupid Me I know.
John