I'm surprised Firefox didn't warn me when I went to the page. Hostile teleco/MITM waiting for HTTP traffic are a real-world way that nation states deliver exploits.
PKI is basically powerless against nation states executing a targeted MITM attack. It does prevent them from passively snooping everything.
You can enable it in the settings.
It did for Librewolf -- what I moved to from Firefox. Self-Signed certs I'm down with, http I'm not, and never will be for any reason. Plain-text data transmissions have no acceptable reasoning.