Actually Firefox 45 on Ubuntu connects with no problems, so I tested with Firefox 47 on a virtual W10, which reports that the certificate is invalid. It also says that the certificate is not trusted because the issuer certificate is unknown (SEC_ERROR_UNKNOWN_ISSUER).
The best thing to do would be to report the issue to the owner of the site, and include the links in my first reply, if you find a way to contact them. I didn’t find any contact inforamtion at https://pamyatnaroda.mil.ru/about/?lng=en
I get a different warning when I go to that site using IceDragon 44.0.0.11. Refer to Image1 below. For those that need to translate, I’ll repeat what the image shows:
Your connection is not secure
The owner of pamyatnaroda.mil.ru has configured their website improperly. To protect your information from being stolen, IceDragon has not connected to this website.
The certificate is not trusted because the issuer certificate is unknown.
The server might not be sending the appropriate intermediate certificates.
An additional root certificate may need to be imported.