That's why I don't understand this either, I copied the Firefox settings profile from Win 7 to Win 8 and it does everything the same except for this one thing. Right now I'm running Firefox 19.02 but the issue occurred in older versions too. My add-ons are NoScript, BetterPrivacy, Exif Viewer, New Tab Homepage, Google Shortcuts, Norton Vulnerability Protection and DownloadHelper.The downloads box in Firefox for Windows 8 behaves the same as Window 7.