A client certificate would be a certificate that the institution provided to you to authenticate your browser to the site so they know it's you. Seems like unusually high security for a university, but who am I to say...

Did your Firefox recently perform a "Refresh" placing your old profile folder on your desktop inside a folder named Old Firefox Data?

If you have that: possibly a client certificate you imported long ago is in there, stored in a file named cert8.db and you could replace your new cert8.db file with that old one (when Firefox is not running, of course). More details if you think that is relevant.

Thank you, jscher2000, but there's been no recent refresh, no Old Firefox Data folder, no cert8.db file. Again, I can reach some bookmarked pages on university website but not others (catalog, staff/student directory, annual reports to president). Any more ideas?

I have McAfee Livesafe set to the usual "run in real time" antivirus, firewall, etc., and I ran a Malwarebytes scan after I saw this. Nothing changed there either. Thanks.

Could you check this setting related to client certificates:

• Windows: "3-bar" menu button (or Tools menu) > Options
• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

In the search box at the top of the page, type cert and Firefox should filter the page. If the "When a server requests your personal certificate" is not set to "Ask you every time" could you select that option and see whether the page triggers a request?

Also you will see the "View Certificates" button. If you click that to open the Certificate Manager and click the "Your Certificates" tab, do you have any listed?

Finally, could you compare with the Windows certificate store. You can call up Internet Options from the Windows Control Panel or from within IE. Click the Content tab, then the Certificates button. In the Certificates window, click the Personal tab. Do you have any listed, especially any related to the University?

jscher2000, thank you--but the answer is no, there is nothing listed under "Your Certificates" in about:preferences and no personal certificates under the Windows certificate store. I'm at a loss.

later that day: jscher2000, thanks again. Problem seems directly connected to spiffy new Ffox 57. I found *.exe for an earlier version on backup, installed, and all is fine now. If Ffox 56 is the old Ffox design I might try to upgrade to that--but I cannot afford to be unable to use my clients' websites.

Many thanks for your time and help--genuinely appreciated. _______________________ To Mozilla generally: Spiffy new designs that leave users in the lurch belong to dear old Microsoft; no need to emulate this feature.

You might ask the school whether other Firefox users are reporting this error message or if you're the only one.

I'm a freelance editor and the school is MIT. They might notice patterns of trouble among inside-MIT users, but they probably use either MIT proprietary software or Chrome. I must solve any technical problems I encounter. I have used various versions of Firefox since 2011 on the MIT website without encountering anything like this (and Ffox since it was part of Netscape). I thought Ffox 56, before Quantum, was OK, but a problem this afternoon sent me back to Ffox 49. I am well aware that the later versions are more secure but I cannot afford to be unable to use my clients' websites--as it is, I have lost substantial time--and I have to be able to concentrate. I can think of no reason why I should be the only one encountering this problem; but not that many people use Ffox any more (14.5% of market last I saw) and techies less so. I am very grateful for your thoughts and suggestions. I will use Ffox until it is no longer possible; that day isn't too far off, it seems. If Mozilla can lose me, they aren't far from the edge. Again, many thanks.