I tried a full reinstall of version 123.0.

Did you refresh your profile when reinstalling and stay signed out of your sync account? An easy way to test a new profile is to install Developer Edition and see if it happens there.

Thanks for the quick response. I repeated the reinstall and added the "refresh profile" you suggested. The problem with Schwab login remains.

Try changing DNS over HTTPS (DoH) to "Off" or "Max Protection" and in Connection Settings change "Use system proxy settings" to "No proxy".

Disable any third-party antivirus or security software, this may require rebooting into Windows Safe Mode (networking enabled).

You can help find the change responsible by running mozregression between version 122 and 123.

Turning off DNS over HTTPS did not help, but subsequently setting "No proxy" solved the problem. Thank you for your suggestions and your time!! A nice boost to my faith in forums for open-source support! Now I'll try to recover the profile that was messed up when I regressed from V 123.0 to V 122.0b9 ...

After a shutdown and restart the problem has returned. I confirmed that the recommended changes in settings are in effect. Any idea what I might have inadvertently reconfigured?

My problem has disappeared after twice repeating zeroknight's suggestions with full reboots and profile refreshes, so I am marking this "solved." Thanks again for the help.