If you change it to false, the download dialog will be displayed again. The key for this is _to_download_panel, whose value is true by default.
Fortunately, there is (still) the possibility for savvy users to restore the old behavior via about:config. Unfortunately, Mozilla has decided not to leave it up to the classic user to decide how downloads should take place, as a preference option for the behavior (i.e., whether new or old) is missing.
