AriaNg's settings are stored in the "Offline Storage"
Aria2's settings are stored in the file like "aria2.conf" and it can only be changed by manually edited that file.

It was designed by the original author, so I can't do anything about it.
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Firefox's notification API has lots of limitations. I cannot create button, progress bar, or something else on the notification.
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Deleting files, running files or using high privilege may cause some security issue.
So it may never have these features.
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The only thing I can do is create a popup of browser action that shows the download items.