I think the thing to do would be to convince Mozilla to include CAcert root certificate in Firefox.
Someone said that this was already being worked on. If additional support from us would help, we would be glad to give it. Can you find out who we should talk to? Much better than trying to convince users to install the SSL cert. They only see the message after they have already had all the error popups and then not bothered to continue.. We're going to change the setup of pages so as to inform the user just when the issue arises.