I have this message in Firefox:

Secure Connection Failed

An error occurred during a connection to board.flatassembler.net. Peer attempted old style (potentially vulnerable) handshake. Error code: SSL_ERROR_UNSAFE_NEGOTIATION

The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
Please contact the website owners to inform them of this problem.

If I hit "Try again" multiple times, I get the page to load eventually, but some pictures or CSS are missing.[/list]

I tried with several versions of Firefox, all successfully and with no warning messages.

What version of Firefox is causing this issue?

Can you attach a screenshot of the Firefox security dialog (it should show us the TLS parameters, as well as who issued the certificate, its expiry, etc).

When page loads successfully, it reports:

CN = Let's Encrypt Authority X3
O = Let's Encrypt
C = US

Owner: This website does not supply ownership information.

Valid date:
From 06 Sep 2018
To 05 Dec 2018

TLS DHE RSA with AES 256 CBC SHA 256 bit keys, TLS 1.2

When page refuses to load, I was only able to see "Connection is not encrypted".

I have tried to poke in various web console tools, but I wasn't able to convince it to show me what particular kind of negotiation it was attempting. If you tell me how to view particular negotiation logs I can provide more details.

The version is GNU IceCat, not actual firefox, but its clone. It can be its bug, actually, but all other sites I visit do not have this problem.