Hi @Lubos,
It seems there is an issue with the Guides section on your site - they are signed with an invalid certificate and Firefox at least refuses to go there due to an untrusted certificate and “www.manager.io has a security policy called HTTP Strict Transport Security (HSTS)”, so no exceptions are allowed.
Through Firefox at least it is not currently possible to view any Guide documents.
Manager uses Letsencrypt which seems to renew every six months or so. It was renewed today, most likely in the last hour! What has probably happened is that your Firefox has the old certificate cached.
I use Firefox too (only browser I will ever use), and the guides are fine for me. Clear your history, cache etc and the guides should work for you.
It started working again here to, without any cache clearing required.
I just put it down to another example of Lubos’ blinding efficiency!