You can temporarily work around the issue and turn off ocsp stapling: enter about:config into the firefox address bar (confirm the info message in case it shows up) & search for the preference named security.ssl.enable_ocsp_stapling. double-click it and change its value to false.
since this will slightly lower your security, it is important however, that after a bit of time when the issue gets resolved by the site (maybe try again in 24 hours), you go back and switch the setting to "true" again!