When you search for a package for the HTTPS version of the site, you are
redirected to the HTTP one:

   $ curl -si 'https://snapshot.debian.org/package/?src=nyancat' | grep -w Location
   Location: http://snapshot.debian.org/package/nyancat/

This makes Firefox with HTTPS-Only Mode enabled confused and unhappy.
It says:

"You’ve enabled HTTPS-Only Mode for enhanced security, and a HTTPS
version of snapshot.debian.org is not available."