The trick is to figure out why Firefox is sending a request to that server. Can you connect the problem to a specific event that is happening in Firefox? Here are a few possibilities, but each case of course will be different:

(A) A page you are loading made the request

(B) One of your add-ons made the request

(C) One of the shortcuts in the built-in Firefox Home / new tab page is triggering the request

Thanks so much for responding. This is beginning to look like the car that squeeks until you take it to a mechanic! I believe the notices appeared when I started Firefox, but I have tried to make it happen again and failed. I even rebooted my PC and tried to make the issue repeat without success.

I tried to clip the list of notices I received from Bitdefender, but couldn't paste it here. I was able to block-copy the most recent one. I hope this provides a clue as to why this is happening. It hasn't happened in 4 hours. Maybe it's best to wait and see if it happens again.

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Suspicious connection blocked 4 hours ago

Suspicious connection blocked 4 hours ago Feature: Online Threat Prevention

firefox.exe attempted to establish a connection relying on an expired certificate to api.faviconkit.com. We blocked the connection to keep your data safe since websites must renew their certificates with a certification authority to stay current, and outdated security certificates represent a risk. Add to exceptions

The last part mentions that the certificate was expired. Normally Firefox wouldn't connect if the certificate was expired, so it seems to be duplicate protection.

When I check right now, Firefox says the site's certificate is valid starting at "Tue, 01 Aug 2023 11:33:02 GMT" so it sounds like they fixed the problem and now everything is good until it expires again.

Thanks again for your help with this. Actually, I don't know exactly what a favicon is, but I read enough on the internet to know that I don't need it and I would like to remove it. Any suggestions about how to do that will be appreciated.

A favicon is a site icon, like the one on this page's tab, or next to a bookmark.

We don't yet know whether the request to that address is triggered by a web page or by an add-on or by Firefox itself because it wants to show a particular site's icon. It's not easy to figure that out without some kind of error message that alerts you to the request

There are add-ons you can use to block web addresses; I don't have any specific recommendations.