New Firefox is complaining about untrusted certificates in even more painful way and the time when CAcert certificates will be widely accepted seems to be far away, so I had to do something with bug tracker certificate. Otherwise I would receive too much of mails about broken certificate (which has been always valid, just signed by CAcert, which is usually not between trusted certificate authorities).
As a cheapest way to deal with this issue I found seems to be StartSSL. The problem is that their root certificate is not everywhere, but at least Firefox contains it. Anyway it is just an attempt to make things work better, if you have any problems with new certificate, just drop me an email.