First of all, thank you very much for your kind words and for taking the time to post, I really do appreciate it!
Yes, there are a few updates but minor: now that Keychain Access allows users to export keychains and certificates, it may be less important to use Firefox in order to retrieve the certificates and keep a copy of the files in Netscape format. I would however still recommend that route, as it is time-tested and the only reliable way to keep a backup that would allow you to start from scratch.
Thawte was at the time very popular for issuing free valid certificates for personal e-mail use, making them an approachable solution. Relying on their services is of course not an obligation and other CAs do exist that will provide you with the files and warranties you need, although you may need to pay for an equivalent service.
As there are as many legal systems and restrictions concerning the use of certificates in various countries around the world as there are baguettes in a French bakery on a Sunday morning, I cannot really recommend someone in particular. I would suggest that you ask a local computer group, your system administrator at work or visit the Apple Discussions. In some countries, advice from a solicitor is also required.