Minor firefox DoS - semi silently polluting ~/Downloads with files

[Georgi Guninski <gguninski () gmail com>]

This is barely a DoS, but since Chrome has explicit protection
against it, we decided to disclose it.

If firefox user visits a specially crafted page, then firefox
may create many files in `~/Downloads`,
The user is notified about this in a small dialog, but there is
no option to stop the downloads.
The potential denial of service is that the user must manually
delete the created files and this might be PITA.

Technically about the PoC:  create non-empty file `xml.doc`.
To force download, add to the page `iframe src="xml.doc"`.
To force creation of new files, add `body onload="location.reload()"`
(there are several other options about this).

[Proof of concept][1]

To out surprise, Chrome is safe from this and it distinguishes
manual download from automated download and this might be because
it is aware about this DoS.

Affected:  firefox 117 on GNU/Linux and reportedly on Windows.
Not Affected:  firefox on android, Chrome, lynx.

[1]: https://j.ludost.net/y3.html
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