This works for me fine -- I go straight to that link and can download stuff. I don't get asked for a password. In order to get their I got the error page saying that the site doesn't appear to support HTTPs, because the cert doesn't match download.proxmox.com (it's for "enterprise.proxmox.com" and a couple other subdomains). If instead I go first to the https version of that site I get the cert error page (because of the mismatch). If I add an exception for the cert then I'm shown an HTTP-auth prompt, and then a 403 page because I don't have a login. Now that I've entered the cert exception for that site I get the behavior you describe.
Bug 1714661 Comment 3 Edit History
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Initially this site works for me fine -- I go straight to that link and can download stuff. I don't get asked for a password. In order to get the page open I got the HTTPS-only error page saying that the site doesn't appear to support HTTPs. This error is detected because the cert doesn't match download.proxmox.com -- it's for "enterprise.proxmox.com" and a couple other subdomains. If instead I go first to the https version of that site I get the cert error page because of the mismatch. If I add an exception for the cert then I'm shown an HTTP-auth prompt, and then a 403 page because I don't have a login. Now that I've entered the cert exception for that site I get the behavior you describe.