What computer did you use to download the ipsw? And have u already upgrade your iOS to 4.3.3 if yes, then
You should first allow hidden file to be showed, you can clic organize >folder and search options. in the popup wondow clic on tab "view" then mark "show hidden files, folders and drives" when that's done go to
C: user > username(your comp user name) > appData > Roaming > apple computer > iTunes > iPhone Softwarw Updates. In that folder you will find your the ipsw which was downloaded by itunes and used by itunes to upgrade your iOS. you can copy, NOT CUT the ipsw to another location like to your desktop. When you start redsnow and it arrive to step 2 browse to your desktop and find the ipsw there.
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