Also, if I simply refresh the login page after receiving this error, and then allow Firefox to resend the data, I'm logged in successfully.

If this is a bug then you need to fill a bugzilla so the people behind Firefox can look into it.

https://support.mozilla.org/en-US/kb/file-bug-report-or-feature-request-mozilla

If this is a bug then you need to fill a bugzilla so the people behind Firefox can look into it.

Its a still a good idea to troubleshoot and discuss the issues here first to see if it is indeed a bug and to minimize duplicates in bugzilla.

What security software do you have?

Boot the computer in Windows Safe Mode with network support to see if that has effect.

• https://www.bleepingcomputer.com/tuto.../how-to-start-windows-in-safe-mode/
• https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/

Thanks for the response, cor-el. I am running only Windows Defender. Safe Mode behavior of Firefox is still the same. My other browsers - Edge, Brave, Opera, and Chrome - do not exhibit this behavior. Just as a reminder... When I open my login link - https://huppbrian.us/tsc-admin - and try to log in with my username, password, and CAPTCHA response, I get the reported error message. If I refresh the page, and allow Firefox to resend my login information, I am logged into my site without issue.

Hello Everyone, I just tried a suggestion I found elsewhere - tell Firefox to "Forget about this site." That worked. Sorry to have bothered everyone.