That's strange, it's valid until 8th January 2021. This doesn't happen for me in Firefox 83 for Mac so that's weird!
"Websites prove their identity via certificates. Firefox does not trust this site because it uses a certificate that is not valid for www.mojeek.com. The certificate is only valid for the following names: dock.shp.mcafee.com, *.dock.shp.mcafee.com
Error code: SSL_ERROR_BAD_CERT_DOMAIN"
Could be caused by the extensive (68k lines) of blocking in my /etc/hosts file, I suppose.
You might want to talk to Mcafee about their classifying your site as dangerous, or just curse them out like I've done when they listed my software as malware. For what it's worth, they do the same to emacssurvey.org so it's not just you :)