If you click through the learn more link, you can see an example of the new warning. Instead of starting the download, it shows a dialog with a warning and ther

If you click through the learn more link, you can see an example of the new warning. Instead of starting the download, it shows a dialog with a warning and there are two buttons, one of which is "Allow download". The second part of this feature is that for <iframe sandbox="..."> elements, regardless of HTTP/S origin, Firefox will no longer allow the iframe to initiate downloads unless the sandbox attribute explicitly includes allow-downloads It appears that there are a pair of preferences in about:config to govern these: dom.block_download_insecure and dom.block_download_in_sandboxed_iframes