Is there an Advanced button on that error page? Can you click it and paste here the certificate?

Screenshot attached. Thank you for your interest.

This is a Kaspersky's cert, not Facebook. See How to troubleshoot security error codes on secure websites. There's an info about Kaspersky AV.

Well, that's the certificate that showed up! I went to the link above, but the information on Kaspersky bears no relation to what I see when I open Preferences.

I wasn't able to quickly find a support article for the latest Mac version. If you do, or a user guide, please let us know so someone can update the support article.

Also, Kaspersky usually filters all HTTPS requests (most search engines and this site, for example) and not just one site, so that's a bit odd. Do you use the parental control product (Safe Kids)?

Thank you for looking, anyway. No kids, no filters. I have exported my bookmarks to Safari, which I don't like, because it's not being blocked. (Famous last words) In Firefox, Kaspersky won't let me go to Kaspersky's website!

"I wasn't able to quickly find a support article for the latest Mac version. If you do, or a user guide, please let us know so someone can update the support article."

Kaspersky's adviser said: "Click on the Kaspersky shield icon at the top right-hand side of the Menu Bar -> Preferences -> Protection -> uncheck the option "Check secure connections (HTTPS)..." and let me know if this workaround resolves the issue."

It did. I'm writing this in Firefox. Thanks to all for help and advice.

HI ALL

I have been blocked out of FACEBOOK for at least a week now, but I believe for a DIFFERENT REASON than above!

I set up a new account FACEBOOK ACCT for an app game. I wanted it separate from my personal FB account I linked it to my Thunderbird e-mail. I was allowed to log-in to FB for about 3 days.

THEN SUDDENLY FACEBOOK has decided my account is a security risk and is NOT ME ! ? They asked me for a photo, which I uploaded, and have not been allowed to log-in since!

Please see the PRINT SCREEN SHOT of the HTTPS:// secured firewall I receive each time I attempt to log-in. This is whether I reset password, go thru Thunderbird. or go thru my original mail server. IT DOESN'T MATTER . .

ANY IDEAS on what is GOING ON ?? Thank You!

@M R Coates, this problem has a different reason than the initial question: it has nothing to do with mozilla nor Firefox.

Please go to Facebook Support.

FYI . . . . I am back onto FB magically. Not sure what happened. I think Google let me back on. But for future reference, I could not simply go COMPLAIN to FB support. WHY? Because I could NOT GET ON my Facebook account!

TyDraniu said

@M R Coates, this problem has a different reason than the initial question: it has nothing to do with mozilla nor Firefox. Please go to Facebook Support.

"Did Not Connect: Potential Security Issue". This suddenly started happening yesterday. Examples:

http://theartshop.com.au/ https://www.startpage.com/ https://www.ptv.vic.gov.au/

But not:

https://www.abc.net.au/news/ https://www.theneweuropean.co.uk/home https://duckduckgo.com/ https://forum.manjaro.org/

If I try an open sites directly it comes up, but if I use the search box I can sometimes get to them.

No problems with the same sites with Opera

Tried to attach screenshots etc but unsuccessful, just goes round and round.