Did you try Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#search104 It works for me. see screenshot

Also, what version of Firefox? What security software are you running? Are your running a home pro version of Windows?

Windows 10 pro Firefox 120.0. No added security when this started happening. Downloaded my subscribed Malwarebytes Pro and ran a scan with no infections found; downloaded and ran MS Malicious Software Removal Tool and found no infections. Not sure if one of the security settings in Firefox 120 is doing this or something malignant is running without being found by security scans. Will try Safe Mode.

Thanks jonzn4SUSE for the suggestion about Safe Mode. Using that mode, I can access Amazon.com and the string I mentioned in my original message does NOT get added to the URL. I can log in per usual. Interestingly, when I return to normal mode, the ?smdm-noredirect=true is added when I go to Amazon.com but I am logged in (something saved during my Safe Mode login?). I will try closing Firefox and starting with a new login attempt to Amazon.

After logging out of Amazon, closing Firefox, reopening Firefox and trying to log in again, I get the same problem and message as described in my original post. I guess I can workaround by opening Firefox in Safe Mode, logging in to Amazon, closing Firefox, opening Firefox in normal mode, and going to Amazon where, apparently, I will be already logged in. But something is happening with Firefox that I need to understand and address. Thanks for any additional ideas.

The workaround described in my previous message works - sort of. Add To Cart doesn't work, while Buy Now does work. Something still is not right. Is the "smdm-noredirect=true" something coming from a new security feature of Firefox 120.0?

One of your extensions might be doing it, try disabling them.

The Firefox extension that sent Amazon to smile.Amazon was still enabled. Disabling it (and then removing it) cleared the problem. Amazon Smile used to make charitable donations based on a small percentage of your purchases. Amazon discontinued the program earlier this year (I think).