When performing a download on Firefox for Android with the External Download Manager option ticked, you get to chose which download manager you would like to use as well as having the option to complete the download with Firefox anyway. This can be particularly annoying when downloading many files one after another. As such setting a default download manager along with adding a small indicator to say a download has been added would be a good addition.
TL;DR: Prevent the need for this popup by adding the ability to set a default download manager.