A sample is like:
<a download="drawn by mx2j (nsh6394) - 0b717a005d5cf62f2c579a9806642591.png" href="https://hijiribe.donmai.us/data/__drawn_by_mx2j_nsh6394__0b717a005d5cf62f2c579a9806642591.png" _pageexpand_="500">Download</a>
And the reason is download property in Firefox only works with the same-origin url.