For me it gives:

"Secure Connection Failed

An error occurred during a connection to www.fanfiction.net. The OCSP server has no status for the certificate. (Error code: sec_error_ocsp_unknown_cert)

   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. Alternatively, use the command found in the help menu to report this broken site."

hello, i can certainly replicate this issue. my guess is that the site is currently implementing measures against the recently published widespread vulnerability that allows webservers with a certain version of openssl running on them to be exploited (heartbleed.com) & is switching their certificate.

an advanced security feature in firefox is picking up this change as the site doesn't seem to be fully updated for this new certificate yet. you can temporarily work around the issue:
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.
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!