Create a new bookmark in your bookmar toolbar, call it download and use this as the URL:
javascript:(function() { var els = document.querySelectorAll('audio, video'); var links = []; for (var i = 0; i < els.length; ++ i) { var el = els[i]; var l = document.createElement('a'); l.href = el.currentSrc; l.download = ''; l.style.visibility = 'hidden'; l.style.position = 'absolute'; l.style.bottom = '0'; l.style.right = '0'; document.body.appendChild(l); l.click(); links.push(l); } for (var i = 0; i < links.length; ++ i) { document.body.removeChild(links[i]); } })();void(0)
This doesn't work in all cases. Some webpages uses that special streaming video format or DRM.