i remember there being bugs in a few mozilla milestones (and the Netscape versions that were built on them) where if the form was a multipart, and you submitted it without actually selecting a file for upload (or selecting a nonexistant file), it would never close the connection properly and just hang like you describe. i don't think CGI creates form tags with enctype="multipart/form-data" unless you explicitly tell it to though. so it's probably not the same bug, but may be related.

anders pearson