Two things have happened simultaneously.
(1) Yesterday the forum was made accessible by https only; this means that (2) is a pain. Eighteen months ago, after a DoS attack on the comic server, the comic and forum were placed behind a CloudFlare cache, to mitigate the effects of such attacks in the future. A while later, I requested https access to the forum for reasons I won't elaborate on here; this was provided, but required removing the forum from the CloudFlare system because the free option (which is being used) didn't support https. CloudFlare has now added https to the free option, and so the forum has been moved back to it, and a new certificate that is not self-signed has been provided. BUT...
(2) Yesterday a major security breach in the old SSL3 protocol was announced (called Poodle). Many servers are now disabling SSL3 to force the use of the more modern TLS protocol. However, some clients still default to SSL3, and may not be graceful about it no longer being available. So, I have one machine at home that can't see the forums today, and another that can; Tapatalk on the iPhone is OK, but not on Android; I have problems with Thunderbird with one university's mail server; there will be a new release of FireFox to handle this shortly (v34); the list goes on and on.
However, I'm puzzled by your seeing SSL certificate errors (I am not, in either of those browsers), because there is now a proper certificate as opposed to the previous self-signed one that would have generated that warning. In this case I suspect that you have been forced to https because of the forum change, but that the CloudFlare server that you are going through still has the old certificate cached. I suggest you either wait for a day or so for it to clear, or delete the self-signed certificate from your certificate store and see if you then get a new one.