Expired SSL cert on nitter.net & cert mismatch on nitter.net
Visiting: https://nitter.net/ in Firefox:
Return:

Websites prove their identity via certificates, which are valid for a set time period. The certificate for nitter.net expired on 5/28/2021.
 
Error code: SEC_ERROR_EXPIRED_CERTIFICATE

Also visiting https://www.nitter.net/
Return:

Websites prove their identity via certificates. Firefox does not trust this site because it uses a certificate that is not valid for `www.nitter.net`. The certificate is only valid for `nitter.net`.
 
Error code: SSL_ERROR_BAD_CERT_DOMAIN