Firefox – “sec_error_ocsp_server_error” when trying to open a HTTPS page

If it's a public CA it is likely an operational error which they would very much want to know about. If it's a private CA or cert then the signer should resign it with a new serial number to get rid of the error (bad form to reuse a serial number... unless you are willing to revoke all certs with that # at the same time). You could download the cert (use openssl as an ssl client and have it save the certificate for you) and then explicitly add the certifiate to the FF root store marked as trusted (http://www.mozilla.org/projects/security/pki/nss/tools/certutil.html) but that might not do the trick depending on where NSS is catching the problem. In any case the right action is fixing (re-issuing) the certificate.

Before anyone blindly turns off that warning by disabling OCSP, double-check that it's really not a client issue. Turning off warning messages is something you only do if you have a VERY GOOD reason and you know what you're doing - or if you are the White House, where "the alarm system had been switched off due to complaints from staff that it was too noisy". However, if you take this issue seriously, you don't do that.

It might simply be that the system clock is slow, because the error message says "contains a date in the future". And it is much more likely that your computer's date/time is incorrect rather than that of the server (that could affect hundreds of clients).

Open a terminal and check your time:

$ date

Restart your NTP service to correct it:

$ sudo systemctl restart ntpd && echo OK

If you don't have the NTP service installed ("not-found" error), install it:

$ sudo pacman -S ntp

Also, make sure it's enabled

$ sudo systemctl is-enabled ntpd || sudo systemctl enable ntpd

With NTP installed and enabled, restart it, give it a couple of seconds to fetch the time and then check the time:

$ sudo systemctl restart ntpd && sleep 30 && date

Then try accessing the website again.

Depending on what desktop environment you use, you may be able to use graphical tools instead of those commands.

And for those who have "fixed" this issue by disabling every security feature until it worked: Please re-enable whatever you have disabled. If you've disabled OCSP, enable it again. Those features are there for a reason.