Are you using the internal download manager of Firefox or an external one?
Using the internal is not a good idea imho, I've been using the classic FlashGet 2 since years, but since it caused some problems with the newer version of Firefox (or Windows 7 x64, no idea) I've switched to Free Download Manager which is actually pretty good too if you configure it to stop nagging you every time you want to dl something.

Anywyas, if you know this is a problem with that extension maybe check it's site for nightly updates or report it to the author?

Personally I'm using the FlashGot extension, but months before I've also had DTA installed since it had the "download movies from youtube" feature before FlashGot.