This happens only in Firefox.
Here's the deal:
I'm using htaccess to restrict my content to passworded users. The content consists of html pages with embedded .mov videos -- the pages and the vids are all in the same directory.
To prevent users from downloading the vids, I'm using the qtsrcdontusebrowser parameter (causes the video to run through Quicktime rather than the browser so that users can't get the video from their browser's cache).
Users now are logging into the site, but when they load one of these pages, they're prompted yet again before the video can play.