If you have time enough and you are watching your download, this is a great enhancement...
But - if you are working in the meantime, suddenly the download window of Firefox pops up, gets the focus, and you enter something, overwriting the original "save as" filename. If you still know what you were downloading - great. If you were only downloading one package - great. Well, and if not: good luck!
Unfortunately, there is no way to ask for the filename first. Firefox only allows to show the save dialog when the content is available as a single file/URL. The only workaround I know is to turn off the dialog in the options.
