There are periodic reports of error messages when trying to login to older Oscar version.
Specifically SSL_ERROR_UNSUPPORTED_VERSION
This is caused by the browser wanting to enforce a newer version of SSL that may not be supported by your Oscar server.
There are 2 solutions: Upgrade the server or adjusting some browser settings.
Adjusting browser settings:
Enter about:config in the Firefox Address bar, then enter ‘security’ in the search bar.
security.tls.version.enable-deprecated true
security.tls.version.min from 2 to 1
Note: This worked with Windows 10, Firefox ESR 128.12.0esr to Oscar v12