I cannot open some SSL-encrypted websites (https) in Firefox right now. I have found out that this is caused by the Online Certificate Status Protocol (OCSP) which confirms the current validity of certificates. All those websites are using certificates issued by cacert.org and their OCSP server (ocsp.cacert.org) is currently quiet busy (“Too many users” or timeouts).
Quick workaround is to temporarily disable OCSP in Firefox (or other affected web browsers). You can uncheck this feature in Preferences → Advanced → Encryption → Validation in Firefox.
Update: Server seems to be working again.