This normally happens when the SSL certificate has been updated. I saw briefly this morning but Firefox is working fine now.
This was made more complex because my work laptop has been misbehaving and has gone for a good kicking in the IT dept and I have a temp laptop so I’m reinstalling stuff I need. Tedious.
@MM0FMF As you say: …
Frome Discourse Support: happens shortly after a certificate has been automatically renewed, in combination with a LetsEncrypt server