Firefox: Secure Connection Failed SSL_ERROR_BAD_MAC_ALERT

Firefox gives:

Secure Connection Failed

An error occurred during a connection to my-ssl-site. SSL peer reports incorrect Message Authentication Code. Error code: SSL_ERROR_BAD_MAC_ALERT

• The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
• Please contact the website owners to inform them of this problem.

But does not give an option to proceed anyway.

Go to about:config. Add my-ssl-site to security.tls.insecure_fallback_hosts:

security.tls.insecure_fallback_hosts my-ssl-site

Seems to be broken in Firefox 80.0.