When are you getting the error? When you enter the login? When you press the 'login' button? Are you using the Password Manager? Many site issues can be caused by corrupt cookies or cache. '''Warning ! ! ''' This will log you out of sites you're logged in to. You may also lose any settings for that website. * [https://support.mozilla.org/en-US/kb/delete-cookies-remove-info-websites-stored Delete cookies to remove the information websites have stored on your computer] {web link} * [https://support.mozilla.org/en-US/kb/how-clear-firefox-cache How to clear the Firefox cache] {web link} If there is still a problem, '''[https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Start Firefox in Safe Mode]''' {web link} by holding down the '''<Shift> ''(Mac=Options)''''' key, and then starting Firefox. A small dialog should appear. Click '''Start In Safe Mode''' (not Refresh). Did this help? While you are in safe mode; Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches. ''[https://support.mozilla.org/en-US/kb/forum-response-disable-hardware-acceleration How to disable Hardware Acceleration]'' {web link}

Thank you. It worked ok when I deleted the password manager entries and keyed them in.

Good to hear. Are you using a password add-on or program? Try re-entering the data into the Password Manager. Is the problem still there?

I don't remember installing anything. I thought autocomplete was a Firefox feature.

There is the Password Manager that stores the login data (only), and there is the autocomplete used for anything else.

I use both.

Your problem could be autocomplete is trying to enter the wrong data.