Show the issue. Are just going to the home page or trying to login? What security software besides MS Defender are you running? Did you try with Firefox in Troubleshoot Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#search

Does this occur when just trying to go to the home page, or is when you follow a more complex link?

Do you notice any difference in a private window? The main differences are:

(1) Private windows do not re-use files Firefox cached in your regular windows

(2) Private windows do not re-use cookies sites previously set in regular windows

(3) Some of your extension might not be running in private windows (you control this individual on a per-extension basis)

First, of course, this just started happening without my making any changes to my system (of course ;-))

It happens when I'm browsing eBay and click on an auction on which I want to see more detail. It appears to be intermittent as this morning, after a fresh bootstrap, there is no problem. As to Firefox, I'm using a "Search by Image" extension, none others and I enabled this extension months ago w/o problem. I'm not a power user and don't know what a 'private window' is. I use Norton anti-virus and whatever Windows 11 provides. Again, no changes (that I know of) here in months and the trouble just started several days ago.

In searching the WWW for a solution, I find that the identical problem occurs in other browsers, too. Ebay is the only site where I have noticed this problem. I suspect eBay has done something to their website.

Perhaps, most notable, is that, this morning, after a fresh power on and bootstrap, the problem is not present.